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

[wagnermaciel]

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