consider nudging people to document new features in the unstable book

Location

https://doc.rust-lang.org/nightly/unstable-book/

Summary

right now, when you add a new unstable feature, unstable-book-gen will parse it out of the source code and generate a stub such as https://doc.rust-lang.org/nightly/unstable-book/language-features/async-trait-bounds.html.

do we actually want this? it seems nice to instead encourage people to write docs for the new features they add. i am imagining something like this (see https://github.com/rust-lang/rust/blob/d3a2440384b54664dddd56e0081ca094c444e869/src/doc/unstable-book/README.md for how this differs from how things currently work):

1. run unstable-book-gen as-is one last time. copy the generated SUMMARY.md to src/doc/unstable-book, so it's tracked by git.
2. change unstable-book-gen to not overwrite SUMMARY.md, merely generate a local copy. then, give a hard error if its generated SUMMARY differs from the one tracked by git.

the second thing requires people to write their own docs, even if they're just a stub. and hopefully while people are at it, they will take the time to write more docs than just a stub.

Read src/doc/unstable-book/README.md and inspect unstable-book-gen, starting by running the generator and comparing its generated SUMMARY.md with the tracked copy. Implement the workflow described in the issue: keep SUMMARY.md tracked, generate a local copy, and report an error when they differ. Done means new unstable features require corresponding entries in the unstable book.