docs: push per-audit docs further into codebase

[woodruffw]

Right now we have a lot of duplicated information between each <audit>.rs and its corresponding section in audits.md. We also have information that isn't shared but should be, such as long-form descriptions in audits.md that should be also included in the codebase so that SARIF and other output formats can include them.

A rough sketch for improving this:

• Move most audit docs to docstrings
• Extract those docstrings with something like the documented crate
• Dump those docstrings and embed them into audits.md via snippets

[woodruffw]

• Extract those docstrings with something like the documented crate

Messed around with this a bit and unfortunately it isn't a great fit -- the Documented trait adds an associated constant, meaning that it prevents object safety (which we need, since we turn everything into a dyn Audit or similar).