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 after the two first dots. Also, instead of "The adapter to use for Propel", we could say "The database adapter that Propel will pick", what do you think ?