/usr/share/sgml/gtk-doc/gtk-doc.cat is in gtk-doc-tools 1.20-1ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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-- DSSL stylesheet ...................................................... --
-- ...................................................................... --
PUBLIC "-//Gtk//DOCUMENT Gtk-doc HTML Stylesheet//EN"
"/usr/share/gtk-doc/data/gtk-doc.dsl"
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