Create CI/CD for documentation build

[RastislavTuranyi]

Might help prevent stuff like #33 from happening, though I suppose that might need its separate testing?

Also not sure whether this will need to be integrated in some way with readthedocs for publishing, or how often we want to build the docs

[oerc0122]

Easiest is GitHub pages every push to master. There're standard workflows for this on the marketplace.

E.g.
https://github.com/oerc0122/castep_outputs/blob/main/.github/workflows/docs.yml

[ajjackson]

Ideally we would

• run doctests and check that docs build on every push to a PR
• deploy to pages on every release
• deplay to some other pages subdomain ("latest", "dev" etc.) on every push to master

[oerc0122]

pytest --doctest-modules may come in handy if not already in use.