Convert content and switch from jekyll to sphinx

[lornajane]

A ReStructuredText project really should have rst website source! Update our content and manual to be rendered by Sphinx and published to GitHub pages. Add some dummy directives to do a "no op" for the stuff that isn't supported/needed when Sphinx renders the manual as HTML.

Things that still need sorting out:

• [x] Run more of the HTML build workflow on every pull request to make it easier to review
• [ ] Test the usage instructions in the README (which possibly should be in CONTRIBUTING, now I think about that more
• [x] The manual pulls in content from other pages that it expects to find nearby, so we need to either bring those files here, include them in the manual, or something else

Followup tasks:

• [ ] Automate getting updated manual rst source and PDF files from the main repo (when? On release?)

We should update the default branch to main and publish from that when we deploy this change.

[lornajane]

To see how this work is progressing, grab an artifact from the build action, extract it locally and preview the site with python -m http.server. If we merge #1245 which removes external dependencies from the manual and generally makes it a bit more docs-like, the result is easier to read (that version of the manual is what's in the branch at the moment, until we automate that part!)