doc/experiments.rst points to deprecated docs location

Since pythonhosted is deprecated, we've moved the public documentation to be hosted by idiap.

Line 139 of doc/experiments.rst needs to be updated to reflect this change:

bob.pad.base/doc/experiments.rst
139:Please refer to the `GridTK documentation <http://pythonhosted.org/gridtk>`_ for more details on how to use the Job Manager ``./bin/jman``.

The various docs versions available for gridtk can be found at https://www.idiap.ch/software/bob/docs/bob/gridtk/.