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

Start by comparing the v18 elevation guide with the current theming page, then inspect the referenced documentation change and the code paths for elevation classes and the mat.elevation-classes() mixin. Done means the theming documentation explains the current elevation support and usage, or explicitly documents its deprecation, with the affected page updated accordingly.

Written by the indexing model from the issue text.