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. 
There's no target given for "reStructuredText" at the end of the file. Should probably be the same as in ``format.rst``.