docs: switch from alabaster to sphinx-book-theme, and misc docs build system maintenance

[consideRatio]

I'm refreshing a few projects documentation build system.

In this case, it also meant leaving the old alabaster theme behind and going for sphinx-book-theme. A preview comparison is available.

• Current docs
• Docs PR preview

Related

• docs: update of readthedocs config and docs/source/conf.py zero-to-jupyterhub-k8s#2909
• docs: add devenv and linkcheck to Makefile and misc details binderhub#1565
• docs: add devenv, linkcheck, and refresh misc config etc repo2docker#1197

[choldgraf]

Definitely an improvement to me!

[consideRatio]

Thanks @choldgraf! Going for a self-merge with that approval to enable me to rebase #353 on it and resolve a merge conflict that otherwise occur.

[choldgraf]

This makes me wonder - does the jupyterhub team have a policy about self merging when there is an approving review? My feeling is that for most PRs, after one approving review we should encourage a merge by anybody unless there are "request changes" present but somebody else, so that you don't need to deal with the decision cost of knowing whether you can self merge

[consideRatio]

Decision cost was a nice concept to relate to!

I agree on that policy I think. Maybe also along with a unburdening clarification for reviewers that that they could do partial reviews ending up in a request for help to understand something or only provide a partial approval that includes a disclaimer that they didn't understand the change details or consequences well enough and ask that another person contribute a review as well.

Pinging a related topic: jupyterhub/team-compass#466

[choldgraf]

That's what I use the "comment" review feature for. I think it's a helpful way to provide feedback while explicitly signaling that you don't wish to formally approve