/usr/share/gtk-doc/html/mx-gtk is in libmx-doc 1.4.7-0ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gtk-doc/html/../../doc/libmx-doc/html/mx-gtk
/usr/share/gtk-doc/html/../../doc/libmx-doc/html/mx-gtk