Improve C++ utilities and backport documentation

This is just a bunch of small changes to the Doxygen docs that

1. introduces a new Doxygen module for our backports and all of the little utilities we have.
2. moves all of those things into the new module.
3. introduces and fills a new Doxygen module for the concepts stuff.

Apart from that, there are a few smaller fixes (mostly missing macro docs and hiding implementation stuff from Doxygen).