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

まず v18 の elevation ガイドと現在の theming ページを比較し、次に参照されているドキュメント変更と、elevation クラスおよび mat.elevation-classes() ミックスインのコードパスを確認します。theming ドキュメントで現在の elevation のサポートと使用方法が説明されるか、その非推奨が明示的にドキュメント化され、影響を受けるページがそれに応じて更新されれば完了です。

索引モデルが issue の本文から書いたものです。