Docs build: use jupytext/mystmd to allow markdown as source instead of ipynb

[jni]

Jupyter notebooks can be annoying to maintain because they are hard to diff, and you can't really edit them in a plain text editor except for the simplest changes. (And still I make mistakes there. 😂)

With jupytext or mystmd, you can store the notebooks in markdown and then they get executed and rendered from that source. The napari/docs repo uses this configuration so we should be able to copy it to here.

[jo-mueller]

Sounds like a great idea!