Fix documentation build

[mdpiper]

Previously, I had been using pandoc to translate the Markdown files in the root directory of the repository into sphinx-friendly reStructuredText files for the documentation. In the move to nox for managing automated tasks, this got lost. It's now breaking. I need to evaluate if this workflow is still needed and fix it.

[mcflugen]

I think the myst-parser sphinx extension should do the trick. I've used it in a several projects, include bmi-wavewatch3.

[mdpiper]

Ah, that would be awesome!

[mdpiper]

Pretty sure this is caused by the md/rst formatting in the changelog when it's updated by zest.releaser. Converting to MyST for the docs should fix this. Or not letting zest.releaser touch the changelog.

[mdpiper]

Fixedf with #68.