Collapsible sections in Jupyter

[slel]

The following packages allow to collapse sections in Jupyter sheets

• when using Jupyter Notebook:
  https://jupyter-contrib-nbextensions.readthedocs.io/en/latest/nbextensions/collapsible_headings/readme.html
• when using JupyterLab:
  https://github.com/aquirdTurtle/Collapsible_Headings

Ideally support that functionality in CoCalc-Jupyter too.
To install:

$ pip install jupyter_contrib_nbextensions
$ pip install aquirdturtle_collapsible_headings