docs-bug(all): Add doc for mixins

[martinboue]

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