Upgrade jupyter-book dep to enable move to Sphinx 5.0+

[SorenSpicknall]

User story / feature request

We recently encountered CI failures related to various sphinxcontrib-[xyz] dependencies that were trying to load in automatically but weren't compatible with Sphinx versions older than 5.0. A Sphinx version is not manually specified anywhere in this repo, but it flows in from the jupyer-book requirements in /docs and in /images/jupyter-singleuser. We currently specify JupyterBook 0.12.0 or ^0.12.2, and 0.12.x doesn't support Sphinx 5.0.

To quickly resolve the build issue without messing around with the JupyterBook version, I pinned several sphinxcontrib-[xyz] requirements in /docs. More cleanly, we should upgrade the version of jupyer-book we're bringing in to one that supports Sphinx 5.0, then unpin these recently pinned dependencies.

Acceptance Criteria

We should use a version of the JupyerBook that supports Sphinx 5.0, and undo the temporary requirement pins in /docs/requirements.txt.

Notes

A relevant StackOverflow thread is here, though not one that's particularly detailed for our needs.