🎨 Theme updates for showing proofs, algorithms, and github previews

[rowanc1]

This incorporates the work on the sphinx proof as well as many other small updates to the theme that have been added by @tavin and me over the past month or so!

There are a few things that I am noticing over here around spacing/margin updates, but overall things are working much better!!

@agoose77 I wonder if you could help with the testing/deploy of this for @mmcky, who is looking for some of these updates. I did a first pass test locally, but it probably needs a manual go through/test. If you are game to release that would be awesome (I am on vacation for the next 1.5 weeks!! ⛱).

This introduces:

• proofs
• github hover links for issues and PRs
• better hover infrastructure for any internal references
• simplified code-block styles
• many subtle style improvements (e.g. around margins and typography)

[github-actions]

👈 Launch a Binder on branch executablebooks/jupyterlab-myst/feat/proof

[rowanc1]

please update playwright snapshots

[agoose77]

@rowanc1 I looked over this and it looks good to me. I think we're at a stage where making these kinds of releases should lean towards new features as a priority, so I am keen to merge this as is!

Small point: I noticed that we support code-cell directives in notebooks. I wonder whether we should prevent that, as it's my understanding that we normally expect these only at the top level, and JupyterLab-myst is highly unlikely to ever support cell-in-markdown (certainly I wouldn't prioritise it, as it's highly non-standard).

Also, do we parse the link previews as Markdown when they're taken from GitHub? I noticed that links in e.g. issue descriptions aren't rendered, and given that we know we're parsing GFM, perhaps we could do something here?

In any case, I'm going to merge this and make a release!

[rowanc1]

Thank you!

Yes, we should parse the body, I considered leaving it out completely, but it is a bit better even there without the parsing for now.

Completely agree about removing or warning on the code-cell, it shouldn't do anything inside a notebook.