Reorganize Tutorials hierarchically

[BalzaniEdoardo]

Give the tutorials a hierarchical structure:
e.g. all tutorials for basis would belong in the same folder, tutorial on model fitting in another.
Another idea could be having tutorial that familiarize with the package separate with more theoretical one. Maybe a session on model extension which has recipes to extend the models we implemented.

[billbrod]

Related to your second point: diataxis is probably a useful framework for thinking about our tutorials and how to structure them.