Add documentation build to weekly cron job

The CI issue with Sphinx 4.0 (#1463) went undiscovered for several weeks, only to rear its head when we wanted to make a PR for something unrelated.

We many want to consider adding the documentation build to the weekly cron job. Similarly, it would make sense to add style checks to the weekly cron job, in case an upstream change to `flake8` breaks those checks.

Separately from that, we should consider pinning the versions of external packages used in the CI testing.