Adding stub to generate Python documentation using Sphinx.

When BUILD_DOCUMENTATION is ON and Python support is enabled, Sphinx can be used
to generate documentation for ParaView's python modules. The generated
documentation needs a lot of cleanup, but this is a start. Now we can edit the
source files to provide useful documentation for the Python API.

Change-Id: I6b7fc1d61fa1cd3d7cead10ffc008034ec311e5e
Merge topic 'python_documentation' into next

9468246 Adding stub to generate Python documentation using Sphinx.