Refs

Add docs for `refs`. The `posts_feed` endpoint response allows including the full content of referenced posts up to a specified max per post. The `post` endpoint supports including referenced posts up to a specified max, as well as an alternate representation that supports pagination of just the referenced post (examples and docs coming soon).

Repost, delete, and cursor posts do not have `content`, only `refs` and `mentions`.

See also tent/tent-schemas#6.