Docs fail to build

[mjaquiery]

Describe the bug
The Sphinx docs are failing to build because the sphinxcontrib.images code hasn't been updated to work with Sphinx 8, and Sphinx 8 is now the current major release.

To Reproduce
Steps to reproduce the behavior:

1. Update main branch to trigger a docs build
2. Go to GH Actions
3. See that Docs build has failed

Temporary fix
We can temporarily fix this by pinning Sphinx version to 7.4.7, which in turn requires using a pip install of Sphinx rather than an apt-get install.

Permanent fix
Once the PR to fix this in sphinxcontrib.images has been merged, we can update to Sphinx 8. #29