Update the Help application links to point to the new documentation tree

The Help application (xwiki-platform-help-ui) pages ships link to xwiki.org documentation that has been migrated to the new   documentation tree under https://www.xwiki.org/xwiki/bin/view/documentation/.

Most of these links still target the legacy Documentation/UserGuide,  Documentation/AdminGuide and Documentation/DevGuide spaces, or extension pages on
  extensions.xwiki.org whose content now lives in the new tree. 

Repoint every link that has an equivalent in the new tree.