/usr/share/doc/libgtkmm-3.0-doc/tutorial is in gtkmm-documentation 3.12.0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libgtkmm-3.0-doc/../gtkmm-documentation/tutorial
/usr/share/doc/libgtkmm-3.0-doc/../gtkmm-documentation/tutorial