docs: update Python 3.11 docs and more

[robrap]

A variety of devstack doc updates, including:

• docs: manual upgrade for Python 3.11
• docs: add note for Demo course is empty in studio
• fix: update readthedocs repo url
• fix: git openedx to edx repo docs

See edx/edx-arch-experiments#578 for future work related to updating our docs.

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I've completed each of the following or determined they are not applicable:

• Made a plan to communicate any major developer interface changes (or N/A)

[robrap]

[inform] This is what I did locally, but my PR still got set up against openedx-unsupported at first, so there are other local changes I probably need to fix master, right?

[timmc-edx]

That's a separate thing we'll need to fix. GitHub knows that we forked edx/devstack from openedx-unsupported/devstack (either before or after the move from openedx) and so it assumes we want to contribute PRs to the upstream. I don't know if there's a way to change that behavior other than asking GitHub to unlink the repo from that "network".

[robrap]

That's a bummer.

[robrap]

I added edx/edx-arch-experiments#578 (comment) to take note of other things I found. I also wonder if I should back out any non-RST changes from this PR so that provisioning is not required?

[timmc-edx]

I found the issue that make docs was failing on and added a suggestion.

But you could also remove the make docs from the end of the provisioning workflow (I don't think that was ever really needed there!) and add docs/ to the paths-ignore of that workflow as well, if you wanted to do some clean there as well.