Restructured docs (and added documentation for multisort)

This PR does too many things.

- Restructured the documentation
  - ``book``: Contains ``introduction``, ``getting started`` and ``creating a basic website``
  - ``reference``: Will contain reference material such as twig functions, content types etc.
  - (we plan also to add `contributing` later, but not in this PR)
- Added Travis configuration - will check for Sphinx errors
- Added documentation for new `multisort` function
- Ported Tiwg extension documentation from [here](https://github.com/sulu-cmf/docs/blob/master/developer-documentation/300-webspaces/twig-extensions.md)
Can you fix it?