Redirect from rule codes to rule pages in docs (#8636)
## Summary
This adds redirects from, e.g., `https://docs.astral.sh/ruff/rules/F401`
to `https://docs.astral.sh/ruff/rules/unused-import`, which are
generated automatically when creating the documentation. Though we want
to move towards human-readable names eventually, I think this is a nice
and user-friendly change (and doesn't require any fancy infrastructure,
since the redirects are handled via a plugin and added client-side).
Closes #4710.
