add docs build to GitHub Actions

[mossmann]

• [x] identify or specify the version of sphinx to be used by readthedocs
• [x] pin docs build to version used by readthedocs: https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html#pinning-dependencies
• [x] add docs build to GitHub Actions so that build errors that will affect readthedocs are caught

[straithe]

It looks like there is no specific version of sphinx that readthedocs uses so I have checked that part of the list off.

[straithe]

I think point 2 is addressed in #1027