Document how to make interdependent targets on different namespaces

The following discussion from !1247 (merged) should be addressed:

• @simon.praetorius started a discussion: (+13 comments)

  Unfortunately, this does not work properly. If one target depends on the other target (e.g. via target_link_libraries), the dependency must be included before. Otherwise I get the fatal error "The following imported targets are referenced, but are missing..." automatically generated in the target file. Not sure how to fix this properly. I have to create the libraries in the right order, I think, and then it works. But this is not the way to want to resolve dependencies.

added buildsystem label

added doc label

For me the question is: where to put this documentation? We could simply add a paragraph in the top of the DuneAddLibrary.cmake file, that should be exported into the html cmake documentation. But maybe this is not the right place - apart from the fact that the new style cmake documentation is not yet properly generated, see !1025

My idea would be to finish the CMake modernization and then add a documentation page on how to effectively use the dune utilities to have modern CMake, including internal/external dependency management. Aside from that, perhaps a note in dune_add_library signature saying that multiple namespaces will likely require the usage of generator expressions.

How about we start a markdown document in cmake/README.md?

Ok, or we can put it here too: doc/buildsystem/dune-common.rst

mentioned in issue #379 (closed)