docs: fix references to github repo

[marquiz]

This relies on the myst parser's (new in v0.19) feature of customizing
external URL schemes with the "myst_url_schemes" configuration option.
We specify new URL schemes "blob" and "tree" which are supposed to be
used for references to blobs (files) and trees (directories) in the git
repo.

This also drops sphinx event hooks ("doctree-resolved" and
"missing-reference"), simplifying the conf.py considerably. They were
useless anyway as they were totally broken and didn't actually fix any
references to repo files.

The downside of this is that the affected references are not shown when
browsing .md files directly from github (as github dosn't handle our
custom 'tree:' and 'blob:' url schemes. However, it should be preferable
to have the cross-references working in the official documentation.

This PR also updates myst-parser to the latest version and pins the versions of
sphinx-rtd-theme and sphinx-markdown-tables for predictable results.

[klihub]

The downside of this is that the affected references are not shown when browsing .md files directly from github (as github dosn't handle our custom 'tree:' and 'blob:' url schemes. However, it should be preferable to have the cross-references working in the official documentation.

The best would be to get both working: keep the links working in the .md files and get them working in the generated documentation as well which I believe will require some python post-genertion fixup logic in the end. Why didn't the fixup in the event hooks work ?