Warning! For the conversion to work, you need to use nbconvert-5.6.1
pip install nbconvert==5.6.1
For installing jupyter notebook extensions (nbextensions)
pip install jupyter_contrib_nbextensions && jupyter contrib nbextension install
Activate the extensions Collapsible Headings and Table of Contents (2).
For converting a notebook with collapsible headings (CH) and table of contents (TOC) still available in HTML, use this command:
jupyter nbconvert --to exporter.exporter.CH_TOC <mylecture>.ipynb --output docs/<mylecture>.html