Upgrade our service documentation to the MyST engine

[choldgraf]

This PR upgrades our /docs site to use the new MyST engine! I believe that this should "just work" - let's see how the PR preview does.

🖼️ Here's the ReadTheDocs preview (for some reason it's not being auto-added to this PR? Maybe because of that action we have that calls it? EDIT: Fixed it by re-connecting the github app)

Plan for this PR

• If somebody wants to review it, that'd be nice but not necessary. I've added @agoose77 but Angus if you're busy please don't feel obligated!
• Unless there are objections I'll merge this on Wednesday.

What I did

Here are a few things that I did:

• Added all the config etc for MyST
• Updated a bunch of sphinx-specific roles (e.g. :ref:) to use MyST link syntax instead
• Removed some custom roles we'd created (e.g. {role}) and just used myst labels for those
• Updated the javascript for our support widget to instead use a myst anywidget - it has a transform that adds the button to every page by a little invisible node it puts at teh end of the page - this is a bit hacky but I couldn't figure out another way to embed JS on each page otherwise.
• Added some footer / navbar / etc.
• Created a new little MyST plugin here: https://github.com/myst-contrib/myst-gui-text and am using that for all our {guilabel} uses.
• Fixed some broken links, moved files, etc.

There were some other cleanups here and there but that's the main stuff I can remember. This is another PR that touches a bunch of files, so maybe Angus could take a quick look and push whatever he changes he likes, and then merge when he thinks it's OK?

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• supercedes Switch to Jupyter Book 2 #307