Add explicit RTD version pin, and protect against Sphinx 8 deprecations.

[freakboy3742]

To ensure a consistent and repeatable build environment, use a hard version pin on the RTD environment.

Also adds a pin to Sphinx on Python 3.9, as Sphinx 8 deprecates Python 3.9 support; and a pin to sphinx-autobuild on Python 3.8.

PR Checklist:

• [x] All new features have been tested
• [x] All new features have been documented
• [x] I have read the CONTRIBUTING.md file
• [x] I will abide by the code of conduct