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).

We do not want one such documentation per module, but instead
we would allow people to use it from their module and get a
documentation generated from all the dependent modules. This
needs some changes to the machine.

This includes the following workflow:

* The cmake module path is globbed for any modules matching *.cmake
* The opening comment is parsed for the following rst directives:
  cmake_function, cmake_module, cmake_variables
* From those, a browsable documentation is generated.

To run properly, a python interpreter and sphinx are needed.