Documentation improvements

[dkfellows]

Working on improving documentation.

• CI building of docs (or at least the Sphinx part)
• Fix errors in doc build
• Add missing method docs

There's still some places where things are double-linked. Those are quite tricky to hunt down when they're in the output of autodoc.

[dkfellows]

That's all the remaining faults that I can fix. The ones that are left (all warnings) are due to the combination of importing the cli stuff from hpcflow.sdk into hpcflow.app and the CLI documentation being generated by click; the place where it's going wrong is just too deep within the nest of generated stuff.

[aplowman]

Excellent, thanks!