WIP/CI: Build docs with single job
The docs build in the CI is taking more than one hour, which seems too much. Based on the logs the time is spent in `waiting for workers...`, which doesn't provide much information. Running in a single job here to see if the logs provide more information on where the time is spent, and see if the job does need to take so long, or if there is any particular step responsible for most of the time.
Seems like there is no single step taking a lot of time. The `Writing output` of each of the hundreds of files is taking like a second, which seems like a lot of time. But no idea what's going on in that step, and the sphinx codebase is so complex and messy that doesn't seem worth checking the details.

FWIW, it doesn't seem to save much time to use multiple workers for the docs. It's like 8 minutes difference in a one hour job.