docs/manual: Link to gitlab.gnome.org
docs/manual: Link to gitlab.gnome.org

* docs/manual/libxml++_without_code.xml: Link to example code in
the master branch in gitlab.gnome.org/GNOME/libxmlplusplus.

GNOME/libxmlmm,GNOME/libxmlmm,GNOME/libxmlmm,GNOME/libxmlmm