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