gh-38444: Moving ideal methods to category of rings
This is moving ``unit_ideal``, ``zero_ideal``, ``principal_ideal`` from
the Ring class to the category of rings
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/38444
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert
