Updating requirements for myst-parser

[choldgraf]

Switching the myst parser branch to origin so that the CI can pass fully.

Supercedes #1192

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Copying issue description from #1192/#1197:

• Updates the requirements file to use the myst-parser instead of recommonmark
• Updates all of the eval_rst blocks to instead use {eval-rst} directives, which are the MyST-equivalent of the same thing
  • This should mean that these docs behave the same was as they did with recommonmark.
  • However, there are likely going to be a few places where the behavior isn't quite the same (e.g., I think recommonmark lets you link directly to headers via [sometext](page#header)). Will need to find these and update them.
• Updates the index page to use MyST markdown syntax, just to give you an idea of what this change will look like elsewhere as well.

[jpmckinney]

Aha, I think I need to give you Maintain privileges.

[jpmckinney]

Also supersedes #1197

[jpmckinney]

Comment moved to #1190 (comment)

[choldgraf]

@jpmckinney shall we add those to-do items to the meta issue here? #1190

Or do you want them addressed before merging this PR? (I think my preference is to merge-and-iterate, so long as there are no bugs or errors being introduced, so that we don't overly complicate each PR).

The other thing we should do is a bit of triage on those issues - which are in-scope for what I'm doing, which are more important than others ,etc.

[jpmckinney]

@choldgraf Indeed - I've moved the comment to #1190; some of the tasks are notes to myself to cover (e.g. search). I'll review this PR with an aim to getting it merged without including further tasks.

[jpmckinney]

🎉

[choldgraf]

Woohoo! Step 1 🙂