include cell execution in documentation of jupyter notebooks

[ltalirz]

The documentation of individual widgets is currently done through the notebooks in the ./notebooks folder.

Currently, the code cells of the jupyter notebooks are not executed (also, in order not to bloat the git repository).
For documentation purposes, however, it would be useful to see the results of the execution.

myst-nb lets you specify whether you want the cells executed (it can even cache the result of the output to speed up the process of building the docs), however for many of the notebooks in question one needs a fully functional AiiDA installation.

@chrisjsewell has been looking into getting something like this to work on readthedocs -
we can either wait until progress is made there or we can execute selected cells and store them in the git.

[ltalirz]

It turns out that the interactive widgets, which would be the outputs of most cells, are not actually rendered in the documentation output, thus making it useless to store the notebook output in the git repo (or executing the cells on RTD).