docs(material/theming): add tokens section to our docs

Documentation Feedback

Add documentation to material.angular.io to list the tokens that our components expose.

Design

There are still some open questions that the team should discuss.

• Should we write a description for each token?
  • Critiques:
    • A unique description for every token may be too much data thrown at the user which the user might mentally filter out when looking through our docs.
    • Many tokens are self-explanatory. It seems silly to write "selected-checkmark-color": The color of the checkmark when the checkbox is selected.
  • A more useful approach may be to provide a subsection that explains terminology that is obscure to most users. For example, the section could describe the following:
    • What state-layer commonly refers to.
    • Listing the different states - active, pressed, etc. Material has a page dedicated to this we could also link to.
• Where should this token list live?
  • IMO under the "API" tab for each component makes the most sense.

This design satisfies the following pain point for our users:

• CSS variables are difficult to identify / discover.

Affected documentation page

https://material.angular.io/components/autocomplete/api

从受影响的 Autocomplete API 文档页面 material.angular.io/components/autocomplete/api 开始，查看组件 API 文档的组织方式。与团队解决有关 token 描述、术语和放置位置的未决问题。完成的标准是：公开的 CSS token 可在适当的组件 API 文档中找到，并且术语指导已达成一致。