[Waiting Code Merge] Introduction chapter for the Form component

This PR is WIP. It contains the sketch for an introduction chapter of a new Form section in the components book. This section is dedicated to:

* explain how to use the Form component without Symfony2
* document the Form component in detail (also for framework users)

I want to achieve both aspects at once by introducing two new configuration blocks:

* "Standalone PHP" (php-standalone) for PHP code that does not rely on the full-stack framework
* "With Symfony" (php-symfony) for PHP code that assumes the usage of the full-stack framework

The differences between code in these two formats is in most cases minimal.

@fabpot: What do you think about this? Could you adapt your Sphinx extension accordingly?

@weaverryan: Do you want to write the accompanying text to the code samples? (where missing)

everyone: Is anything essential missing in this chapter?