Add P2 site references to update site
Similarly to how the product references the Eclipse update site and Oomph update site for the particular version we're using, we should add them to the update site as well, in case people are using the update site directly, rather than via our product.
See http://www.lorenzobettini.it/2021/09/eclipse-p2-site-references/ for more information.
Edited by Dennis Hendriks