Install Autotools Integration's Example 'Makefile.am'
The manual says:
First copy the Makefile.am from the examples sub-directory of the gtkdoc-sources to your project's reference documentation directory (e.g. docs/reference/). A local copy should be available under e.g. /usr/share/doc/gtk-doc-tools/examples/Makefile.am.
-- GTK-Doc Manual/Project Setup/Integration with Autotools/Integration with automake
No example Makefile.am is installed.