Implement some functionality for dune_cmake_sphinx_doc for custom docs.

In dune-testtools we built a sphinx cmake documentation which is customized
quite a bit. With the core functionality being in dune-common, it is hard
to maintain it twice. I have therefore added some options to the dune-common
doc infrastructure, that are beneficial downstream.

You can now:
* provide custom rst sources
* provide a custom sphinx-build conf file
* limit the documentation to the current module, instead of adding all requirements automatically

The defaults of all those parameters result in an unchanged behaviour
(compared to before this commit).