PRODUCTION PROCESS OF A MULTI-LAYER PANEL WITH AT LEAST ONE INSERT AND PANEL THUS OBTAINED DESCRIPTION The present invention refers to a process for producing a multi-layer panel equipped with at least one insert and to the multi-layer panel thus obtained. For some time multi-layer panels (also known as "compound" panels) with visible inserts have been present on the market, generally used in the field of interior'design and furnishing for the creation of equipped walls, furniture, doors, dividers, display cabinets and other, in which the inserts are positioned in holes formed after the assembly of the layers that make up the panel. Usually, indeed, first of all the layers of the panel are assembled, then holes of a suitable shape are made on the multi-layer panel thus assembled and finally the inserts are fixed in such holes through mechanical clamping (for example screwing) or through an adhesive substance. One of the greatest limits of a panel thus produced consists' of the progressive yielding of the attachment of the inserts above all due to the differential deformations and/or dilations to which the inserts and the panel body are subjected, due to atmospheric agents, humidity, temperature, mechanical stresses or other. Moreover, in conventional processes it is often not possible to mount the inserts flush with the surface of the panel because the inserts have a holding frame that also penalises the aesthetics of the finished product. The technical task proposed of the present invention is, therefore, that of realising a process for producing a multi¬ layer panel with at least one insert and a panel thus produced that allow the aforementioned technical drawbacks of the prior art to be eliminated. In this technical task a purpose of the invention is that of realising a process for realising a multi-layer panel capable of providing a solid 'and long-lasting attachment of the inserts. Another purpose of the invention is that of providing a process for realising a multi-layer panel that allows the aesthetics of the finished product to be enhanced and that does not impose substantial restrictions on the choice of shape of the inserts. The last but not least purpose of the invention is that of realising a multi-layer panel with inserts that can be applied in various sectors of the prior art. The technical task, as well as these and other purposes, according to the present invention, are accomplished by realising an assembly process of a multi-layer panel, characterised in that at least one first hole is made in a first layer of said panel, an insert is introduced into said first hole, and then the layers of said panel are assembled so as to define a first and second holding portion of said insert facing each other on opposite ends of said first hole. In practice, the panel is realised using pre-perforated layers . The inserts occupy the empty spaces defined by the holes of the layers and remain blocked inside the panel with the application of the last layer. The present finding also refers to a multi-layer panel characterised in that it has at least one insert introduced in a corresponding first hole of a first layer of said panel, and a respective first and second holding portion of said insert facing each,other on opposite ends of said first hole. Other characteristics of the present invention are defined, moreover, in the subsequent claims. Further characteristics and advantages of the invention shall become clearer from the description of a preferred but not exclusive embodiment of the process for producing a multi-layer panel equipped with inserts and of the resulting panel according to the finding, illustrated for indicating and not limiting purposes in the attached drawings, in which: - figure 1 shows a front view of a multi-layer panel in accordance with a preferred embodiment of the invention; figure 2 shows a sectioned elevated side view of an enlarged detail of the panel of figure 1; and figure 3 shows a sectioned elevated side view of an enlarged detail of a second embodiment of a panel in accordance with the invention. Equivalent parts in the drawings shall be indicated with the same reference numeral. With reference to the quoted figures, a multi-layer panel is shown wholly indicated with reference numeral 1. The multi-layer panel 1 has at least one insert 8 introduced in a corresponding first hole 5 of a first layer 2, and a respective first and second holding portion 10 and 11 of the insert 8 facing each other on opposite ends of the first hole 5. The inserts 8 of the panel 1 can be any shape and can be arranged according to any predetermined ornamental design, and in figure 1 for example they are rectangular and are arranged in two parallel horizontal rows. The panel 1 described here has the first layer 2 arranged between a second and a third layer 3 and 4. The first holding portion 10 is formed from the surrounding wall of at least one second through hole 6 formed through the thickness of the second layer 3 and over the housing hole 5. For each insert 8 of figure 2, the second holding portion 11 is offered directly by the surface of the third layer 4 facing the hole 5, whereas for each insert 8 of figure 3, the second holding portion 11 is formed from the surrounding wall of at least one third through hole 7 formed through the thickness of the third layer 4 and over the hole 5. If, as illustrated, the hole 5 of the layer 2 is a through hole, the panel 1 must comprise a minimum of three layers so that the holding portions for the inserts 8 can be formed from the layers adjacent to the layer 2. Otherwise, if the hole 5 is blind in accordance with a possible variant of the invention, in other words if such a hole 5 does not entirely cross the thickness of the layer 2, the panel 1 could have a minimum of two layers with a holding portion being defined directly from the bottom of the hole 5 of the layer 2. Therefore, the present invention limits the number of layers of the panel to two whereas it does not of course set a limit to their maximum number. In the embodiment of figure 2 the insert 8 consists of a first extreme part 14 matching the second hole 6 and having a first face 19 aligned with the outer surface of the second layer 3, a second extreme part 15 matching the third hole 7 and having a second face 20 aligned with the outer surface of the third layer 4, and an intermediate part 16 matching the hole 5. In the embodiment of figure 3, on the other hand, the insert 8 consists of a first part 17 matching the second hole 6 and having a first face 21 aligned with the outer surface of the second layer 3, and a second part 18 matching the hole 5. Advantageously, the first and second portion 10 and 11 face each other on the whole of the perimetric zone of the housing hole 5. In a different embodiment it is possible to foresee that a single panel 1 carries inserts 8 of different shapes, that the second and third holes 6 and 7 have different shapes and/or sizes not only with respect to the housing holes 5 but also with respect to each other, that the first and/or second portion 10 and 11 face each other only on a portion of the perimetric zone of the housing hole 5. The layers of the panel 1 are preferably joined with adhesive material applied to the contact surfaces between the layers themselves. The layers of the panel are preferably made from cardboard, wood, plywood, expanded material, plastic or metal just as the insert 8 can be made from wood, plastic, glass or metal. The insert 8 can be full or hollow (as illustrated) , decorative or functional, for example having means for connecting or supporting an external element at the panel, etc. The panel can be used to realise furniture and/or furnishings, a door, a wall, a false ceiling, a divider, an aeration system if the inserts are hollow, etc. The panel can also be used to realise a soundproofing system, and in this case the panel can be associated with a sound absorption material or one or more layers of the panel itself can be directly made from sound absorption material. The panel can be used to realise a lighting system in which the inserts carry the lights and the electrical connections are passed through channels suitably formed in the layer or in the intermediate layers of the panel. The inserts can in turn carry electric or electronic circuits for emitting or recording sounds or images, for detecting smoke or the temperature, etc. The process for realising the multi-layer panel in accordance with the invention consists of making at least one first hole in a first layer of the panel, introducing an insert into the first hole and then assembling the layers of the panel so as to define a first and second holding portion of the insert facing each other on opposite ends of the first hole. With reference to the preferred embodiments of the invention, in particular the process in briefly the following. One or more holes 5 are made on the first layer 2, one or more second holes 6 are made on the second layer 3 and possibly one or more third holes 7 are made on the third layer 4. Each insert 8 is introduced into the corresponding hole or into one from the second and the possible third hole 6 and 7 intended to lie over the hole 5 of the insert 8 at the time of assembly. The .layers 2, 3 and 4 are assembled arranging a corresponding second and possibly third hole 6 and 7 over each of the holes 5 and they are joined together through adhesive material on the contact surfaces. In practice, it has been noted how the process for realising a multi-layer panel with inserts according to the invention is particularly advantageous in that preperforated layers are used and the inserts are applied during the assembly of the layers and not in a subsequent step to such assembly. In this way it is possible to realise an effective mechanical binding that locks the inserts in position in a firm and long-lasting manner. The process for realising a multi-layer panel with inserts and the resulting panel are susceptible to numerous modifications and variants, all of which are covered by the inventive concept; moreover, all of the details can be replaced with technically equivalent elements.