This commit merge the `CONTRIBUTING.md` and `doc/CONTRIBUTE.org` file
into a new `CONTRIBUTING.org` file. It also refactors the content to be
more organized and make its navigation more goal-oriented.
Github displays a link to the `CONTRIBUTING.*` file when people are
opening new issues or proposing new PRs. This file is important because
it is the entry-point to contributions guidelines for most of the users.
The current setup is non-optimal, even broken, as it adds a level of
indirection, and points to a contributing file that has broken
links (due to the new documentation format). The possible drawback of
the proposed solution is that I'm not sure if it is possible to include
it into the new online documentation as it is not in `doc/` folder.
The other possibility is to keep a small `CONTRIBUTING.md` file, as it
is now, pointing to the new documentation system (once online), but I'm
preferring the proposed solution for the following reasons:
- People that are willing to contribute will probably open Github
first (for forking, creating PR, etc), not the online documentation.
- It has one level of indirection less when people click on the
guidelines guide from a new issue/PR.
- `CONTRIBUTING.*` is by convention a special kind of file on github, so
it's valid reason to break the rule and not having it in the `doc/`
folder.
