Missing packages when building the docs

[janfb]

When building the most recent docs I noticed several problems:

Upon running jupyter nbconvert --to markdown ../examples/*.ipynb --output-dir docs/examples/ && jupyter nbconvert --to markdown ../tutorials/*.ipynb --output-dir docs/tutorial/ as described in the wiki release workflow I get:

• on macOS I get ModuleNotFoundError: No module named 'jupyter_contrib_nbextensions
• after installing it via pip, I get a new one: ModuleNotFoundError: No module named 'notebook.base
• solution: install a lower notebook version: pip install --upgrade notebook==6.4.12

Maybe we do need to add those to the dev requirements, but we should update the wiki or move this to the actual contribution.md

[Baschdl]

It apparently worked as it is right now in @famura's tests in #1063 (CI run) with basically the same command:

https://github.com/sbi-dev/sbi/pull/1063/files#diff-ea3ea8c9932adc7ba8161ceda844fedd43b006848ef1140c050cbd7ea0788a18R74-R78

Maybe it's once again dependent on the Python version. Python 3.8 installs an old version of notebook and you executed it with a newer Python version which then also installs a newer notebook version. Sounds quite plausible.