Reorganize the documentation

[mwouts]

I would like to reorganize the documentation and e.g

• put the emphasis on Jupyter Lab rather than on Jupyter Notebook
• test and document whether the Jupytext menu is available in Jupyter Notebook provided through nbclassic
• Reduce the README to a very short introduction (with a quick demo of to the py;percent format)
• expand the collapsed parts of the README about e.g. how to open text notebooks with a double click, and move them to the core of the documentation