Completely re-reading the controller chapter

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | 2.3+
| Fixed ticket | n/a

Hi guys!

This follows #4427, but only for the controller chapter (I will open separate PR's for `from_flat_php_to_symfony2`, `routing`, and `templating`).

This is more than just updating for the best practices. This is about face-lifting these important chapters after several years of us as a community realizing what's more important and what's less important. More notes:

- I removed some information about using the `defaults` to pass additional information to your controller from this chapter. This is too early and we have this now: http://symfony.com/doc/current/cookbook/routing/extra_information.html

- I removed `ContainerAware` information. This is not the right spot... and maybe nothing is. I don't see much use-case for this - it seems like you'd either extend the base Controller or register your controller as a service.

- I kind of want to remove or move the forwarding stuff. I can't find a real use-case for doing a forward from one controller to another. It adds a lot of overhead and I don't see any situation for it.

Thanks!
Is embedding controllers into templates a subject that should be covered here? I think this rather belongs the "Templating" chapter.