Documentation overhaul

* Definition of some documentation standards in doc/15_development/documentation and gave a list of some to dos in the section "Contribute by..." (see https://github.com/s3v/shinken/blob/doc-fixes/doc/source/15_development/documentation.rst)
* Shortened doc sub directory names
* Reduced levels visible in toctree
* All links have been updated to the path/file#anchor format
* Code previews have been fixed
* Updated the files to follow the other documentation standards