Move the docs in another folder

[jcelerier]

Right now they're in src/, I think they should go in docs/ as it is a very surprising organization. Sorry for missing that during review!

[samamou]

I could change it, but by default mdbook init does this
https://rust-lang.github.io/mdBook/cli/init.html

• change docs to src
• Add a introduction.md with TOCs, different from the readme.md