meson_options: disable gtk_doc by default
We don't need to build documentation for gtk-doc using subprojects, not by default at least, and our 'auto' option would override the subproject 'disabled' default in libnice's case.
We don't need to build documentation for gtk-doc using subprojects, not by default at least, and our 'auto' option would override the subproject 'disabled' default in libnice's case.