A transition towards use of MyST instead of rST?

I've never really become comfortable with rST, but in the past Markdown wasn't a good option for documentation in projects because it lacked the ability to reference other files etc in a good way. This has changed with MyST though.

MyST with Sphinx is very similar to using rST with Sphinx. The key difference is the use of Markdown syntax instead of rST syntax, where the Markdown syntax has been complemented with rST features described as _roles_ and _directives_. A key point with the use of MyST, is that Sphinx will be agnostic to if MyST has parsed markdown files into the format that Sphinx processes, or if it was an rST parser. Due to this, as @TomAugspurger pointed out:

> Things like the dask-sphinx-theme will still work just fine, regardless of whether the source is in rst or markdown.

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In the JupyterHub org, we have a few projects that have successfully transitioned MyST markdown with Sphinx instead of rST with Sphinx, and it has been a great success in my mind. For me, it has become easier to maintain docs in those projects and I believe it helps our Jupyter ecosystem contributors contribute to docs a bit easier as most of anyone working with Jupyter knows Markdown already.

By opening this issue, I hope to spark a discussion about the use of rST, MyST, or potentially other alternatives. If there is support for it, I'd also be happy to put in work to pilot a transition from rST to MyST in the dask/dask-gateway project.

### Related

- [JupyterHub Helm chart use of MyST](https://github.com/jupyterhub/zero-to-jupyterhub-k8s/tree/main/doc)
- [MyST Markdown reference](https://myst-parser.readthedocs.io/en/latest/)
- https://github.com/dask/dask-gateway/issues/427