Add documentation for "Fonts" endpoints in 6.5

[getdave]

If the Fonts Library feature ships in 6.5 then it would be extremely useful to have the new endpoints documented. Namely:

• WP_REST_Font_Collections_Controller - endpoint for predefined lists (i.e. "collections") of font families.
• WP_REST_Font_Families_Controller - endpoint for parent "family" which typically (but not always) contains 1 or more font faces (i.e. the individual fonts).
• WP_REST_Font_Faces_Controller - endpoint for individual font faces within a family.

[mikachan]

Noting that https://github.com/WordPress/gutenberg/issues/57616 is a good starting point for these API docs, and https://github.com/WordPress/gutenberg/issues/57980 for Font Collections.

[swissspidy]

The API docs in this repo are mostly automated AFAIK, just follow the readme to create the new docs once the release is out.

This should be documented in the handbook if that's not already the case.