docs-bug(Guides): theming custom components

### Documentation Feedback

Hey!
Since a few versions ago I noticed the section for theming your own component was removed:
https://v18.material.angular.dev/guide/theming-your-components
Since v19 this was removed.

I am currently trying to assess the effort to migrate our library to M3 and realized the page that would explain how to theme custom components is gone.

The theming page (https://material.angular.dev/guide/theming) does not cover it, only focuses on your own components.

Not only the docs but it seems we lack tools to create those styles without having to copy/paste a bunch of functions on the core/tokens folder.

### Affected documentation page

https://v18.material.angular.dev/guide/theming-your-components