Developer Documentation

Beginning of a developer documentation. The goal of this PR is to move the content of the wiki to a proper documentation hosted with the others documentations. At the end we would like to have a real documentation in one unique place.

Any help is welcome.

P.S Please keep in mind that it's a Developer Documentation and not only an Extension Developer Documentation, the goal is to explain how the different parts works/are designed, how to use them and how they can be used by extensions.

TODO:
- [ ] CSS change to handle <table> and <code> better
- [ ] Extensions
          - [ ] https://wiki.phpbb.com/Developing_Extensions
          - [x] https://wiki.phpbb.com/Extension_meta_data
          - [ ] ...
- [ ] Migrations
          - [ ] https://wiki.phpbb.com/Migrations
          - [ ] ...
- [ ] ...