Install libgtk-3-doc by entering the following commands in the terminal:
sudo apt update sudo apt install libgtk-3-doc
Description:
documentation for the GTK+ graphical user interface library
GTK+ is a multi-platform toolkit for creating graphical user interfaces. Offering a complete set of widgets, GTK+ is suitable for projects ranging from small one-off tools to complete application suites. . This package contains the HTML documentation for the GTK+ library in /usr/share/doc/libgtk-3-doc/ .
Homepage: https://www.gtk.org/
Version: 3.22.30-1ubuntu1
Section: doc