This link was introduced after A new article about XWiki and it seems it was noticed that it was dead too there.
It seems that it’s been rewritten and is available at https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/Tutorials/Creating%20A%20Contact%20Manager%20Application/
I’ve fixed the broken link, see https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/DataModel/?viewer=changes&rev1=64.2&rev2=64.3&
Thanks