Test documentation with the oldest supported Sphinx version

[AA-Turner]

As we've discussed at various points recently, it would be of some use to have an automatic test that the minimum Sphinx version that we advertise in Doc/conf.py actually works. See PR for a new GitHub Actions job to test this on all documentation changes.

A

Linked PRs

• GH-103903: Test the minimum Sphinx version in CI #103904
• [3.11] GH-103903: Test the minimum Sphinx version in CI (#103904) #103948

[AlexWaygood]

Thanks @AA-Turner!