Notebooks in zarr-python

[mzjp2]

Looks like the majority of lines-of-code in our Zarr-Python repo is Jupyter notebook files. Thoughts on moving them out to a separate repo (zarr-benchmarks?) to keep the zarr-python repo tidier and hopefully get GitHub to recognise that it's a Python repo, not a Jupyter Notebook repo. It'd also keep things just generally tidier and enforce seperation of concern.

I'm also not sure if this is an issue I should make in here or in zarr-python (or somewhere else). let me know if I've put it in the wrong place, and I'll close it and migrate it to the right place.