Hide cells both in jupyterlab and myst rendering

[bsipocz]

Experimenting with a bit back and forth with Angus, I found a solution that apparently works for both usages we have. Draft as I still need to run a grep/sed to change all. And will also need to follow-up with a bit of documentation cleanup in the mystmd docs.

#myst-education-2026

[bsipocz]

Test failures are not related to changes here and this is ready for review and merge.

With the review I mean @jkrick or @troyraen - please have a look at the rendering to see if you are happy with the hidden cells, e.g. in here: https://circle.scientific-python.dev/output/job/701f33d8-d06c-4171-9a38-d0054519489e/artifacts/0/_build/html/openuniverse2024-tde-light-curve

Other option is that we can hide input or we can hide output, right now I went with hiding the whole cell.

[jkrick]

Thanks for working on this. I think this is an improvement over having the code cells in the notebook. I don't like that the hidden cell in the rendering says "Notebook ...". Do we have the ability to change that text phrase to be something of our choosing? ie., "Function Definitions" or "Code Cell" or ??