Naida Goro authored Mar 03, 2022 and Reinhard Biegel committed Mar 31, 2022

This commit fixes a merge artifact, which led to two doc folders `./doc`
and `./sim/doc` to exist and evolve in parallel.

- The content of both folders are manually merged into `./doc`
- Folder `./sim/doc` has been deleted
- The outdated DoxyGen documentation has been removed (again)
- Sphinx warnings have been fixed