GH-127133: Remove ability to nest argument groups and mutex groups

[savannahostrowski]

This PR removes the ability to:

• Call add_argument_group on an argument group
• Call add_argument_group or add_mutually_exclusive_group on a mutually exclusive group

We have been warning users of this deprecation since 3.11.

• Issue: Raise exception when nesting argument groups and mutually exclusive groups #127133

---

📚 Documentation preview 📚: https://cpython-previews--127186.org.readthedocs.build/

[savannahostrowski]

I think this is the right error type but let me know if there's a more appropriate type.

[savannahostrowski]

@iritkatriel Perhaps you can take a look at this one if you've got bandwidth, since you added the original deprecation warnings ☺️

[savannahostrowski]

Hmm, let me double-check that there aren't other formatting simplifications we can make now as a result of dropping this.

I took a look and ran branch coverage with a couple of different programs, and nothing stood out to me, so I think it's fine to proceed with this and then tackle any other potential refactorings in separate PRs.