Draft: Add a wrapper for dune-common-config.hh that is always present in ther source directory.

Summary

With !1262 (merged) we have introduced new config.h files slit into a private and public part dune-common-config.hh and dune-common-config-private.hh. The first one is located in [build]/include, the second in [build]/include_private.

The name <dune-common-config.hh> does not tell me that it is a generated file, nor where to search for it. It is hidden in the build-cmake/include subdirectory. Who will understand that and find it there just based on the name? Also, I have to look up the precise naming whenever I want to include it, is it dune_common_config.hh or dune-common-config.h or dunecommonconfig.hh like the regular naming convention in dune? I either need to know what the form of this config include is, or need to read deep in the cmake files, or just copy the include from one of the few other places where it is used. And do I need to include the private or the public config file? To be honest, I don't know until now.

This MR adds an auxiliary header file dune/common/config.hh that just includes the correct dune-common-config.hh and adds a documentation that this file is auto-generated.

See also the discussion in !1313 (comment 132265)