Add GitHub workflow to build docs when docs change

[cgay]

Needs to exit non-zero when there are serious errors. Not sure whether sphinx currently does that.

Should only run when .rst files or dylandomain files or sphinx conf.py files change.

Ideally we could upload the built docs to opendylan.org/ci/<branch-name>/ or something like that, for viewing.

(And we should update libraries-test-suite.yml to only run when non-doc files change. Different bug; dropping it here.)