docs: Convert to MySTMD

[lundybernard]

Convert the docs to MySTMD

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Parent ticket: scientific-python/scientific-python.org#846
PR: #792

[henryiii]

Probably best done after scientific-python/scientific-python.org#846?

[lundybernard]

At this stage, we're mostly concerned with converting each subdomain site to mystmd to verify that it produces satisfactory results. We want to identify issues in the process, and missing features, and take the time to update mystmd to meet our needs.

Once everyone is satisfied with the results of the change to mystmd, we can deal with the task of deploying the updated site. Hopefully we can do this asynchronously, and avoid a huge update which requires merging multiple PRs simultaneously. The discussion on that part of the process probably belongs in the parent ticket.