Upgrade Sphinx dependencies + readthedocs python version

[jsstevenson]

Feature description

We currently pin Sphinx dependencies and use Python 3.11 on readthedocs to build. However, some of these dependencies are incompatible with newer python versions, so if we need to update our readthedocs build, we'll need to increment them. Good idea to do so, regardless.

In particular, we probably want to pin Sphinx 8.1.3.

Use case

building docs

Acceptance Criteria

• update all Sphinx dependencies
• update template .readthedocs.yaml

Proposed solution

No response

Alternatives considered

No response

Implementation details

No response

Potential Impact

No response

Additional context

No response

Contribution

Yes, I can create a PR for this feature.