Change way to enumerate exercises and quizzes

Nowadays we enumerate exercises and quizzes by Module, e.g. [Exercise M3.01](https://inria.github.io/scikit-learn-mooc/python_scripts/parameter_tuning_ex_02.html) is the first exercise of the third module and [Quiz M6.03](https://inria.github.io/scikit-learn-mooc/ensemble/ensemble_quiz_m6_03.html) is the third quiz of the Module 6.

Enumerating is quite ok for a hard progression, but the issue comes into play for additional/optional modules, for instance [Exercise 01](https://inria.github.io/scikit-learn-mooc/python_scripts/feature_selection_ex_01.html) is the first exercise of the Feature Selection Module. Moreover, the clustering module to be introduced by #836 can be taken at either after M3 or at the end of the supervised modules, which makes less clear if we should name its exercises and quizzes as M4.01, ..., or M8.01 ...

My proposal is to stop using numbering and rename the contents (not the files) to reflect their Module subject, e.g. "Exercise tuning 01", "Wrap-up quiz clustering", etc.