Build a transverse documentation website of the whole organization via Mkdocs

[Baptouuuu]

This PR will publish a new documentation at http://innmind.github.io/documentation/

The main goals of this documentation are:

• put in writing the design principles used throughout Innmind
• provide a way to progressively learn from low to high abstractions and how they fit together

This documentation does not intend to be an extensive one. Each documentation is still the place to learn all the capabilities of each one.

[Baptouuuu]

The experience building this documentation has been better than the traditional markdown files in GitHub used in other projects as it provides ways to add extra information without getting in the way of the main content.

All packages with an extensive documentation will, in time, migrate to documentations built with mkdocs.