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

[PowerKiKi]

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 5753ce2, 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

[PowerKiKi]

An alternative solution would be to change the redirection to point to https://material.angular.io/guide/system-variables#elevation, which contains some information, but still lacks information about class usages. Are they deprecated as suggested by #30055 (comment) ? Are they not-deprecated but not recommended either ? whatever the case, they still need to be documented in some way.