Configuration system refactor: documentation

Update the documentation according to the new Propel2 configuration system.
See pull request 527 on Propel2 repository https://github.com/propelorm/Propel2/pull/527

*Note*: I've added a new layout ("configuration") which is the same as "documentation", with a jquery-ui tabbed panel, to show the various formats of the configuration file.
There is a missing `.html` in front of each link. Maybe you can update it in your `minor-fixes` branch and remove the update of that list here (or simply update it in this pull request if it's easier for you).