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.
The correct anchor is `#supported-formats`. Also I guess you want to use "e.g." instead of "i.e." here ; the latter is generally used with an exhaustive list.