Revamped the documentation about "Contributing Docs"

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

This is the first step into completely revamping the *Contributing* section documentation. The main changes are:

  * A much more friendly and welcoming tone.
  * Thorough and step-by-step explanations that assumes no previous knowledge from the reader.
  * The use of real examples to illustrate the explanations, instead of using abstract concepts. 
  * The old [Managing Releases](http://symfony.com/doc/current/contributing/documentation/overview.html#managing-releases) section has been completely removed. This has nothing to do with contributing documentations. It's just documentation for a process that only concerns to doc maintainers.
That's right, but I see no reason to introduce *new* occurences of this string.