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.
Uhm...they're old tickets based on the previous vcs. Honestly, I don't know how to link them. And I don't know if the old vcs is still working, too. Maybe we can delete this line.