doc: specify Doxygen OUTPUT_DIRECTORY as an absolute path
Currently, doxygen is run by ninja in the top-level build directory,
therefore the "doc" folder is always created there. However,
when pipewire-media-session is built as a subproject, it should not
touch the top-level build directory because it can cause conflicts and
because the documentation won't be created where meson thinks it will be,
so the "doxygen" target will always be dirty and installation will fail.