Investigate moving to Retrolab (JupyterLab with classic notebook interface) rather than Jupyter notebook

One day the classical notebook will be gone. More details [here](https://github.com/jupyter/enhancement-proposals/pull/79) for example. Full disclosure: I haven't read it :wink:.

What need to change:
- environment to install Retrolab dependencies
- local instructions to launch Retrolab
- binder link to use RetroLab
- other things?

There may be a few issues:
- a while ago there was no way of opening .md or .py files as notebook by default you needed to right-click -> "Open as notebook" in the files view. **Edit**: actually this may have been fixed see https://github.com/mwouts/jupytext/issues/271#issuecomment-982181041. Cases which could be an issue:
+ full-index.md but the script generating it could easily be converted to create a notebook with a single markdown cell instead
+ I think it is quite rare to open `.py` and `.md` files from the files menu otherwise ...
- does it affect FUN? Ideally we don't need have to change much and FUN can stay with the classical notebook interface if they want
- other things?