DOC: contributing/documentation: remove

This documentation page does not contain much relevant information,
and it suggests to use nox to build the documentation, whose AFAIK is
not really used by any of the regular contributors, thus it is likely
to stop working without anyone noticing.

Author: dnicolodi

docs/contributing/documentation.rst

@@ -8,5 +8,4 @@ Development
 
 .. toctree::
 
-   documentation
    release-process