Improving "The Quick Tour" tutorial

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.4+
| Fixed tickets |

This pull request tries to improve the current [Quick Tour](http://symfony.com/doc/current/quick_tour/) tutorial. Instead of updating its contents, this is a complete revamp of the original tutorial:

  * Some sections have been drastically simplified (e.g. the installation section)
  * A few sections have been completely removed (e.g. the security section)
  * Most of the notes and tips about advanced features have been removed

Before adding comments and corrections for this proposal, please keep in mind that this tutorial is for **the absolute Symfony beginner** and that it should provide just a brief tour of the most important Symfony features.