Switch to MyST-Parser from recommonmark

[jpmckinney]

recommonmark requires RST in your Markdown, or the use of the custom markdown directive from this repo; MyST Parser instead just parses the content of note, csv-table, etc. directives as Markdown, which is what you want.

recommonmark sometimes requires the custom AutoStructifyLowPriority; MyST-Parser doesn't interfere with translations.

For translations to work, a fork of recommonmark needs to be used. MyST-Parser just works.

recommonmark is basically unmaintained (old PRs to fix translations, etc.). MyST-Parser is very responsive.

[jpmckinney]

recommonmark will be sunset in about a month: readthedocs/recommonmark#221

[robredpath]

Thanks for this @jpmckinney - we'll take a look!

[robredpath]

We did this