gh-37341: Avoid algebra in polynomials
One avoids using `Algebra` and `CommutativeAlgebra` in the polynomial
classes.
And the general method `is_commutative` is delegated to the categories
of Rings and CommutativeRings.
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/37341
Reported by: Frédéric Chapoton
Reviewer(s): Martin Rubey
