Updating omero_plone links to point at OME 5 product pages

This PR updates the conf.py so all the omero_plone extlinks go to the new OME 5 pages once they are live and updates the Feature list link in the OMERO docs side-menu.
I've also removed reference to out of date product pages under OMERO 4 so we don't have to redirect or replace them. 

N.B. This will cause broken links until the OME 5 pages are made public so can be excluded from the build if necessary.