create mdbook setup for RFCs

[lsf37]

It'd be nice to render accepted (implemented, active, and deferred) RFCs as mdBook so they are more readable and easier to browse.

For this we'd need to:

• [x] add toml book config etc
• [x] generate a summary markdown file, e.g. like in https://github.com/rust-lang/rfcs/blob/master/generate-book.py
• [x] add a GitHub action to generate
• [x] set up GitHub pages for the repo to publish
• [x] link to generated mdBook from the README and doc site

It does not look like we'd need a separate text directory like in the rust repo, it should be fine to put everything directly into src for the book.