Add automated tool to ensure new feature gates are documented

[camelid]

This would be part of a larger effort to automate documentation updates. The
issue is that updates to the compiler may require updates to documentation, but
it's not as simple as just updating a doc comment; you may need to:

• Update the rustc-dev-guide
• Update the unstable book
• Update the reference
• etc.

The goal for this first issue is to just add a check that would be run in CI to
ensure all feature gates have documentation in the unstable book. Quoting from
@jyn514 on Zulip:

I think we should nibble at this a little at a time instead of trying to get a fully general solution, because the fully-general solution isn't possible without you specifying the dependencies yourself and I really hate that in build systems

See the relevant discussion here on Zulip.

[jyn514]

This would fit as a new check in tidy I think. Not sure if it already parses feature gates, I know it parses error codes though.

[albertlarsan68]

cc #103548

[jyn514]

this should also apply to unstable (-Z) flags.