Remove the Symfony2 documentation

As of today, the PropelBundle is not up to date to provide Propel 2 in a
Symfony application so let's remove the doc for now.

Feel free to revert it once the bundle will be ready for this version.
Added back missing links on the index documents

Added back links which weren't backported (e.g. Silex cookbook)
Move each chapter to match its number

Since the validators chapter has been removed, all the previous chapter
numbers don't match their order.