Added shortcut methods for controllers

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#11593)
| Applies to    | 2.6+
| Fixed tickets | #4666 

This commit is associated to https://github.com/symfony/symfony/pull/11593 that adds new shortcut methods to controllers.

If anything is wrong with my commit or this description, please tell me and I will fix it as soon as possible. I'm glad I finally try to help this awesome project.

Ah, actually, since this is the first example showing `redirectToRoute`, we should put the commented-out example showing `redirect()` and `generateUrl()` (the code block I added in my comment here: https://github.com/symfony/symfony-docs/pull/4109/files#r22320539) to this code block, not the one below.