Consider switching documentation to MyST markdown

[choldgraf]

MyST Markdown is a new flavor of markdown that provides all of Sphinx's functionality from within Markdown (as opposed to reStructuredText). You can do anything in MyST markdown that you can do in rST with Sphinx. I've found that it makes documentation much more pleasant to write, and also much easier for others to contribute to since markdown is more commonly-known than rST.

Also of note is that rST and MyST markdown can co-exist...there's no need to switch entirely from one to the other.

How do folks feel about moving a few pages of the nbconvert docs to MyST markdown to see how it looks. We can treat that as a pilot to then decide whether to begin a broader migration to MyST over time.

cc @willingc