* Doxygen had a longstanding bug [0] where it would use `dot` even if
not configured to do so. Due to this behaviour our config magically
worked.
This bug is fixed in 1.9.2 therefore we need to explicitly enable
`dot` support in order to keep existing functionality.
* Enables WARN_AS_ERROR to catch mistakes.
* Adds a version string to the header to easily identify which commit
the docs are built from.
[0] https://github.com/doxygen/doxygen/issues/7273
