Simplify and explain learning examples

[jardinetsouffleton]

The learning CP examples are hard to understand for newcomers. It would benefit them if all examples available in this repo were as simple as possible so that they can start playing with their own CP problems as fast as possible. To that end, we should refactor all learning CP examples to make the examples as legible and intuitive as possible. The explanations for the examples can be provided as code comments or as formal documentation in the SeaPearl.jl project.