Revamped the documentation about "Contributing Docs"
Added missing link
lways use absolute links instead of relative for internal doc links
Switched another relative link into an absolute reference
Added a note about not using relative internal links in the doc
Added lots of fixes suggested by reviewers
Added another bunch of fixes suggested by reviewers
More and more fixes and improvements
Removed two highlight formats which are "experimental" and not
used by end users
Fixed all the errors found by Ryan
feature #4223 Revamped the documentation about "Contributing Docs" (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
----------

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.

Commits
-------

88ddbb1 Fixed all the errors found by Ryan
baa06b5 Removed two highlight formats which are "experimental" and not used by end users
9bcd33d More and more fixes and improvements
15f3c94 Added another bunch of fixes suggested by reviewers
ff66e94 Added lots of fixes suggested by reviewers
991ff6f Added a note about not using relative internal links in the doc
79375ae Switched another relative link into an absolute reference
4751eb9 lways use absolute links instead of relative for internal doc links
53c3a16 Added missing link
723603c Revamped the documentation about "Contributing Docs"