meson/doc: Fix doxygen config

* 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