Plonk book improvements

[markulf]

There are some merge artefacts in
https://github.com/ZK-Garage/plonk/blob/master/plonk-book/src/chapter_2.md

I also noticed some formatting problems.

I would also like to have more cross pointers between the book and the code, and between the book and the plonk paper and other papers.

David suggested to use a separate branch for documentation, to avoid too many small pull requests into master.

[mathcrypto]

Using a different branch for documentation is fine but at some point it would be nice to have them merged into master.
Could you please refer to the issues you found @markulf ?