Documentation: Doxygen Build Process Refactor

This MR rewrites the doxygen shell script to:

• Be simpler (less change of directories), and annotated
• To use correct sed substitution (#cmakedefine) for config.h.in, which is consistent with the introduction of cmake

It also adds doxygen support to cmake as a custom target, which one can call like:

cmake --build ./build --target doyxgen

from the project root.

Added @LecrisUT to confirm my cmake additions are ok

Tagging @lang-m as he may want to adopt this change in Gitlab CI, once merged.

Update

CMake support for doxygen is added separate of the shell script. This avoids any need to mock headers in the source folder, and results in build-specific documentation. The use of rsync and how to correctly depend on CheckSum should be discussed.