Fixing Documentation "Book" section

Your contribution docs claim that you use the Symfony Documentation Standards. So well, I decided to make your docs follow it :japanese_ogre:. When reviewing, I also fixed lots of other things.

That's why I'm going to split this up into multiple PRs, each fixing one section of the documentation.

I also want to discuss the "Symfony" chapter from the Book. Imo, it's useless to somewhat copy the Quick Tour from the core documentation. In the CMF, we just said something like "This documentation assumes you have a working knowledge of the Symfony Framework. If you haven't, start by reading the Quick Tour first." This would fix what I call the SRP of documentation: It's the responsibility of this documentation to document the Sylius project, not to document the Symfony application.  However, if you think something else, feel free to keep it :)