Add markdown support to docs

Okay, let's get this show on the road. This MR adds the myst_parser plugin to Sphinx to enable us to write our documentation in Markdown rather than reStructuredText.

For background, we have discussed moving away from RST in favor of Markdown due to MD's relative simplicity and people's greater familiarity with it. RST is a great system, but more of our community will be familiar with Markdown, so it makes sense for us to use the tool that will allow for easier collaboration.

In time all documentation will be ported to Markdown (and updated along the way).