Documentation reloaded

The official documentation has been started a long time ago and has evolved aside Modoboa.

According to the important number of questions we received about installation, upgrade and other configuration issues, we realized this documentation was not as clear as we expected. To fix that, a huge rework of the layout and the content has been made and the result is now available!

We hope you will find it better than before and as always, we wait for your feedback because without it, we won't be able to make this documentation even better.