Changes

Initial CMake based Doxygen doc generation. (#1237)

  - This adds CMake support for the `Doxygen` based documentation generation. All relevant options which were set in the `Doxyfile` used for the `Makefile` based build are set for CMake accordingly. I hope I have not missed any.

    The image generation is changed from `.png` to `.svg` and support for interactive `svg`s (zoom and pan) are enabled.

  - The generation is not enabled yet. Just the functionality is added for now.