Update the URLs of API_DOCUMENTATION

Update the hrefs for the following libraries and tutorials at API_DOCUMENTATION

GTK+/Glib Libraries

• GLib
  from https://developer.gnome.org/glib/stable/
  to https://docs.gtk.org/glib/
• GIO
  from https://developer.gnome.org/gio/stable/
  to https://docs.gtk.org/gio/
• GDK
  from https://developer.gnome.org/gdk3/stable/
  to https://docs.gtk.org/gdk3/
• GTK
  from https://developer.gnome.org/gtk3/stable/
  to https://docs.gtk.org/gtk3/

Xfce Developer Tutorials

GTK+ 3 Tutorial (Official)
from https://developer.gnome.org/gtk3/stable/gtk-getting-started.html
to https://docs.gtk.org/gtk3/