gh-36906: various fixes in quadratic_forms (ruff, pep8, error links, etc)
this is fixing various minor details in `quadratic forms`
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/36906
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert