Add Doxygen configuration for creating api documentation

This is just a proposal for now, the documentation "improvements" are mostly stubs to create some initial pages. For now this is just meant as a starting point to get some api documentation going.

In order to create the documentation install doxygen and run it in the base directory, it will put the generated docs under ```docs/html```.

An example how it looks for now can be found here: https://in-verted.de/~leonhard/obs-docs/