Updated help menu item.

Point to the main page for the plug-in library on the Protege wiki,
instead of a specific page that only lists plug-ins compatible with the
current version of Protege.  The issue is that most plug-in developers
don't keep version information current on the Protege wiki, so the list
of version specific plug-ins is often outdated.
Updated help menu items.

Updated link to release notes page for P5 beta, and updated link to
citing instructions to go to the new website instead of the wiki.