[WIP] Revamp the documentation

I don't have to explain the idea behind this PR, I think. SonataAdminBundle is known to be very hard to work with, mostly due to it's "not that great" documentation. There is lots of duplication and it's more written in a "complete reference style" than a nice-to-read book.

That's why I've choosen SonataAdminBundle as my christmas project this year and with this PR, I want to give you the christmas present: **A complete new, easy to read, guide for beginners.**
In the end, this will replace most of the hard to understand things in the reference guide. I'm also planning to remove the great amount of duplication between the adapter bundles.

To be honest, I failed. I wanted to have it finished before Christmas, but I didn't have enough time. That's why I present you the first 3 chapters here, while working on the other chapters (I think it's going to be around 5 chapters).

I'm welcoming all feedback on this PR.

I wish the complete Sonata team, it's contributors and all other people reviewing this PR a merry christmas and very happy new year! :christmas_tree: :santa: :snowman: :fireworks: 
Composer is now able to require the latest stable version if you do not specify the second argument