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.

It's worth noting that the Sphinx version is stuck at 7.3.7 at present because of an incompatibility with sphinx-csv-filter; see crate/sphinx_csv_filter#49 for details.

PR Checklist:

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