docs-bug(all): Add doc for mixins

### Documentation Feedback

Add documentation for all the mixins and explain what they are used for and what the parameters are. This includes:
- Component mixins (examples: mat.button-theme, mat.button-color, mat.button-typography, mat.button-density, ...). I think this should be documented under the new styling tab on each component page. For now, there is only the new `overrides` mixin.
- Global mixins (examples: mat.get-theme-color, mat.define-theme, mat.core, mat.theme, mat.elevation-classes, mat.app-background, ...).

### Affected documentation page

https://material.angular.io/components/categories

Start with the affected documentation page at material.angular.io/components/categories and review the existing component styling documentation, especially the new styling tab and the overrides mixin. Inventory the component and global mixins listed in the issue, then define what documentation each needs for its purpose and parameters; done means the relevant mixins are documented in their intended component or global locations.

Written by the indexing model from the issue text.