docs-bug(core): Missing documentation about elevation since v19.1

### Documentation Feedback

In v18, we had https://v18.material.angular.io/guide/elevation that documented in depth what is, and how to use, elevations in Material.

Since 5753ce24a3e3bf743a9fc9f471eff46b85a7817b, and v19.1 that content is declared to have been moved to https://material.angular.io/guide/theming. However that new page does not even include the term "elevation" at all. So we cannot even know whether classes such as `mat-elevation-z2` are still supported, and even less how to make them work (via `@include mat.elevation-classes();`).

If this is something that has been deprecated (which does not seems to be the case when reading code), then it should at least be mentioned as such. Otherwise the original documentation should be restored.

Would you accept a PR that restore previous documentation, but included on the new https://material.angular.io/guide/theming ?

### Affected documentation page

https://v18.material.angular.io/guide/elevation

Commencez par comparer le guide elevation de v18 avec la page actuelle sur le theming, puis examinez la modification de documentation référencée ainsi que les chemins de code des classes elevation et du mixin mat.elevation-classes(). Le travail est terminé lorsque la documentation sur le theming explique la prise en charge actuelle d'elevation et son utilisation, ou documente explicitement sa dépréciation, avec la page concernée mise à jour en conséquence.

Rédigé par le modèle d'indexation à partir du texte de l'issue.