Remove old and unsupported sphinx GHA

[ocefpaf]

Nothing to see here (yet). Just testing if we can fix the docs build and/or update the sphinx build here.

This one is ready for review.

[ocefpaf]

@paigem this improves a bit the doc situation here by:

• removing the stale ammaraskar/sphinx-action GHA that relies on an outdated Dockefile with old sphinx
• add dependabot for the GHA only, to keep them updated
• install and builds sphinx via the Python GHA + requirements.txt file (easier to maintain, edit, and debug)
• adds taminomara/sphinx-problem-matcher GHA to keep the sphinx inline comments (untested)
• removes app.connect("source-read", rstjinja) that prevent sphinx from running.

That last step is odd b/c the current approach doesn't error out on it but I could not get an error free build with rstjinja. I'm not sure that is actually used so we need input from the folks that implemented that to be sure we can remove it. It was added in #293

Or, maybe there is just a malformed jinja somewhere and rstjinja is working as expected, not sure.

TL;DR this is not ready for merging 😬

[ocefpaf]

OK, the problem was not rstjinja but a double documented file, the hpc.md and hpc.rst. I removed the md version and, if that is OK, this PR is ready for review.

[TomAugspurger]

Thanks @ocefpaf! It does look like sphinx-problem-matcher is working: https://github.com/pangeo-data/pangeo/pull/913/files#diff-9956fdac1df3f4fd0af96bed18d51583a4ab55b933b4efb6a91999f4fb4698a9 (nothing too serious, so we can fix those later).