Some modifications

[GuillaumeGen]

It contains a bunch of details which should enhance the user experience (at least to my mind), and 2 major modifications:

• Gazelle is considered completely out of the scope of this tutorial. There was 2 mentions of it in the README, one in the mention of the difference with the Google codelab and the other one in a TODO. Furthermore, running gazelle modified all the solution files, so I do not think that it is something we want to mention here. (But it is worth a specific tutorial, which probably already exists somewhere).
• Add a small "solution" to the last exercise.

[GuillaumeGen]

This pull request was doing too many different things. The diff was too hard to read, and it would creates discussion on very different topics to take place on the same thread. I will open multiple PRs instead.