/usr/share/doc/glibmm-2.4 is in libglibmm-2.4-doc 2.56.0-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libglibmm-2.4-doc
/usr/share/doc/libglibmm-2.4-doc