[WCM] Revamped the Quick Start tutorial

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | master (2.6+)
| Fixed tickets | #4360

This is a total revamp of the Quick Start tutorial. Following the recent trend in the Symfony world, everything is  simpler now. The main changes are:

  * The tutorial requires PHP 5.4 because this way everything is simpler. By the way, PHP 5.3 is unsupported since August 2014 and it's extremely dangerous to keep using it.
  * We use the `AppBundle` and the other changes that will introduce the standard edition soon. There are some pending PR.
  * Some advanced features, such as HTTP caching, have been removed. This tutorial is aimed for the total newcomer and it doesn't make sense to confuse him/her with things not needed at first.

There is one important thing missing:

  * The section about installing Symfony is pending of the new Symfony installer, specially for Windows users.

And one last comment:

  * Someone, which I cannot remember, commented a few weeks ago that maybe we should get rid of the different versions of this tutorial. I agree that the tutorial aimed for newcomers should be unique to avoid confusions and it should always explain the latest and greatest Symfony features. I've created this PR against the master branch for that reason and I ask the doc managers (@weaverryan, @WouterJ and @xabbuh) to think about removing all the previous versions of this tutorial.

Your proposal is better, but I think it doesn't convey the meaning that I was thinking of. It's not only that Composer manages `vendor/`. The thing is that you should never ever touch anything on that directory. This is exclusively managed by Composer. Any suggestion to better reword the original phrase?