Mention tesing juptyer notebooks?

[Remi-Gau]

Came across this package to run jupyter notebooks with pytest.

https://pypi.org/project/nbmake/

Playing around with it to test someone else's demos:

https://github.com/Remi-Gau/GLMsingle/blob/fb9f0a9781c8d575abfd7742505593b8006ad876/.github/workflows/run_demos_python.yml#L42

Maybe worth mentioning.