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 an extra space at the end of the first sentence and you can say "at the object creation time". :-)