Add a pre-commit hook for jupytext

[gtca]

After #9 and #12 are merged.
See https://jupytext.readthedocs.io/en/latest/using-pre-commit.html.

[grst]

What would be the purpose? I just .gitignore the ipynb files and jupytext anyway syncs them every time a notebook is saved.

[gtca]

scanpy, muon, scvi-tools all use rendered .ipynb notebooks. As I understand, we want to keep this opportunity, especially for the large workflows, where we don't want to or can't rerun them regularly. So we can't just gitignore them?

The idea behind this was, I believe, to also have rendered .ipynb files. Or maybe just providing the guidance for setting up jupytext might be enough. I guess you could lead the best practices here if you're already using it.
Also, don't we need to sync .md and .ipynb files also in case .md files were modified? I can see how this is not an issue if .ipynb files are ignored.

[grst]

I understand why in some cases the ipynb files are needed, but in that case why not just add the ipynb files and not bother with jupytext at all?

It could be nice for code-reviews though...

[grst]

Best to implement this together with #40.

[grst]

Closing in favor of #40