Add meta descriptions to rule pages (#13234)
## Summary
This PR updates the `scripts/generate_mkdocs.py` to add meta
descriptions to each rule as well as a fallback `site_description`.
I was initially planning to add this to `generate_docs.rs`; however
running `mdformat` on the rules caused the format of the additional
description to change into a state that mkdocs could not handle.
Fixes #13197
## Test Plan
- Run `python scripts/generate_mkdocs.py` to build the documentation
- Run `mkdocs serve -f mkdocs.public.yml` to serve the docs site locally
- Navigate to a rule on both the local site and the current production
site and note the addition of the description head tag. For example:
- http://127.0.0.1:8000/ruff/rules/unused-import/
- https://docs.astral.sh/ruff/rules/unused-import/
