Render the JSON schema on the site

[woodruffw]

This is purely a "nice to have": we could use json-schema-for-humans to convert limbo-schema.json into Markdown and embed it as a page.

[woodruffw]

Played around with this a bit, and there are some blockers: json-schema-for-humans doesn't accept Draft 4, which is the JSON Schema format that Pydantic v1 emits. So we'll probably need to unblock #56 first.

[woodruffw]

WIP: #143.