Changes to Getting To Know Lean

[RomanJos]

This PR regroup any future changes to not spam, its a continuation of my previous PRs

The changes done are aimed at fostering discussion into what to actually change in the book, the work already done is massive and my changes may be irrelevant, incorrect, etc as I am a complete beginner so please take everything as a feedback rather than an actual request for changes. A PR allow me to write everything how I see it instead of explaining how I would see them etc.

Again, thank you for the work you've done so far, its quite big!