Repo tooling: add testing to ensure each rule has a corresponding .md

[JoshuaKGoldberg]

Following the thread from #833 (comment): it'd be great to have testing make sure each rule is documented. In typescript-eslint we make sure rule docs exist at a certain path and follow a certain format (typescript-eslint > docs.test.ts).

I'm up for contributing this if you'd like it!

[ljharb]

I agree, and yes that’d be great - although it’s possible that #901 might provide this already. @bmish?

[bmish]

Yes, since eslint-doc-generator checks all rule docs, it will already fail when a rule doc is missing.

[JoshuaKGoldberg]

Nice, love it!