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

从 material.angular.io/components/categories 上受影响的文档页面开始，查看现有的组件样式文档，尤其是新的样式选项卡和 overrides mixin。盘点 issue 中列出的组件 mixin 和全局 mixin，然后确定每个 mixin 针对其用途和参数所需的文档；相关 mixin 在其预期的组件位置或全局位置完成文档记录即表示完成。

由索引模型根据 Issue 内容生成。