Port docs to Markdown/MyST

[econchick]

While I will always have a soft spot for restructured text, I find myself making mistakes a little too often (e2fe333f5a1f29163741c5f77d8ba02dfcd4b94a , 53b1907d0a6360546a39c4527667aad8db63909e). I'd like to port the docs to Markdown, while still using Sphinx with MyST.