Test documentation builds against the development branch of Sphinx

We recently ran into some problems due to breaking changes in the release of Sphinx v9 (#23, #24, #25).

To make it easier to find and fix problems due to upstream breaking changes before they get released, it would be helpful to test building the plasmapy_sphinx docs against the development branch of Sphinx as part of CI. This would help us raise issues in upstream repositories so that extensions could get a head start on compatibility fixes too.

This is low priority since we have a workflow to test PlasmaPy documentation builds against the
development branches of Sphinx, plasmapy_sphinx, and a few other upstream dependencies. That workflow will let us know about breaking changes ahead of time, assuming I remember to pay attention to it. 😅 But, building the plasmapy_sphinx documentation using the development branch of Sphinx would make it easier to address breaking changes before they make it into an official release.

Start by locating the existing CI workflow that builds the plasmapy_sphinx documentation and inspect how Sphinx dependencies are installed. Add coverage for the development branch of Sphinx, then verify that the documentation build runs successfully in CI and exposes upstream breaking changes.