add section about how rustdoc uses static analysis

[lolbinarycat]

inspired by #2298 (comment) and my own confusion with eslint

rustdoc currently uses two tools for linting, eslint and tsc (as well as universal tidy checks, like line length), and we don't currently explain how to use them in much detail, it would be nice to have a separate section for this, instead of mixing this info in with other stuff.

[lolbinarycat]

es-check is another static analysis tool we use.

I think "run all the rustdoc static analysis stuff" should be split into a separate "testsuite" under bootstrap, instead of just grouping it under ./x test tidy.