Spring clean-up of filenames and folders

Some filenames are less than great. Not a short-term priority but this may worth looking at it at some point.

Examples:
- 02, 03, 04 not relevant anymore on the modelling pipeline notebooks. For example, hyper-parameter tuning (04_something) is in another module now
- 02_metrics in the middle of
- maybe some other things https://github.com/INRIA/scikit-learn-mooc/blob/master/jupyter-book/_toc.yml
- different folders python_scripts, jupyter-book/*, does it still make sense, since JupyterBook is quite central now, should we move jupyter-book to the root of the repo or at least have most of the content inside jupyter-book folder?
- some folders in `cross_validation` are in Module 2 and others in Module 7 for historic reasons
- maybe figures can be cleaned up

Probably we should get rid of numbers in the filename in general. They were originally used to navigate the notebooks when giving the course, but with #111 (still to be done) this may not be needed anymore.

**Add more items in this issue** when you bump into them