/usr/share/gtk-doc/html/gladeui-2 is in libgladeui-doc 3.20.0-2+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/gtk-doc/html/../../doc/libgladeui-doc/html
/usr/share/gtk-doc/html/../../doc/libgladeui-doc/html