Remove outdated or excessive info in doc header

As discussed on IRC a while ago the current header of the docs seems a little excessive. Most other gtk-doc generated headers (e.g. for Gtk3, GLib etc) only contain a generic reference to where the latest version can be found online, e.g. "This document is for the GTK+ 3 library, version 3.24.20 . The latest versions can be found online at https://developer.gnome.org/gtk3/. If you are looking for the older GTK+ 2 series of libraries, see https://developer.gnome.org/gtk2/."

From my point of view we could drop everything we currently have there (incl. the license info, which is provided in other places anyway) and replace that with a stable link to https://developer.xfce.org/xfce4-panel/index.html

Thoughts? @skunnyk @kevinbowen