Improve doc build; fix Read the Docs config

[mdickinson]

Our Read the Docs build is currently failing. The proximate cause is that I removed the docs optional requirements when things got migrated to pyproject.toml, but I suspect that even with that fixed other things would have gone wrong.

This PR reworks the doc build a bit to make it more Read-the-Docs friendly:

• Hook up the apidoc build in the Sphinx configuration file, so that we only need the usual sphinx-build -b html step to build the docs; the api documentation generation is taken care of for us. This ensures that the API docs will be included in the Read the Docs build.
• Move the documentation requirements to docs/requirements.txt, and pin versions. (Without this, RtD will use Sphinx 1.8.6. See https://docs.readthedocs.io/en/stable/build-default-versions.html#external-dependencies for more info.)
• Update the RtD configuration file.
• Update the doc test build workflow and etstool.py for the above changes.

Test build on RtD: https://readthedocs.org/projects/envisage/builds/19891791/

[corranwebster]

LGTM - one comment