Demo workflow with ma-tutorial

[rossbar]

Grabbed ma-tutorial proposal from numpy#29 and used it here as the basis for demonstrating a sphinx-based workflow.

The source file is in text-based notebook format (md) converted with jupytext (i.e. jupytext -k python3 --from notebook --to myst content/tutorial-ma.ipynb)

[rossbar]

The workflow I have set up here looks like this:

1. The user submits the tutorial in text-based (md) notebook format. If the user submits something in .ipynb format instead, we can either:
   • Set up a CI service to auto jupytext it over to .md (overkill IMO, and not set up here), or
   • A reviewer can convert it for them: it's a one line jupytext command (see above)
2. Any data needed by the tutorial can go in the data/ directory.
3. CI services create the notebooks and subsequently the sphinx site. I've used CircleCI in this case just because I'm familiar with it, but GH actions or any other CI service would work.
4. Reviewers can use the standard GH interface to comment, suggest changes etc.
5. The run/rendered output is rebuilt on each push, and can be seen via the build artifact (see the checks).

By no means is this the only workflow. In fact I think it's best for the site to be independent of the notebooks, but I'm still experimenting with different approaches.

Re: download/launch links - this site uses the sphinx-book-theme which is based on the pydata-sphinx-theme used for the reference docs, so the feel should be similar, but has those nice widgets builtin on the top for downloading notebooks/launching them on binder. Note that I didn't bother properly configuring the "launch" links, so they don't actually work, but it should be straightforward to do.
Alternatively, if we want the theming to be exactly the same as the reference docs, we can use the pydata-sphinx-theme instead and modify the templates to include download/binder links.

[rossbar]

I didn't realize this was still open - we're well past this point :)