Repair or drop the links to the documentation from the updater

Probably due to the privsep steps we’ve taken, the links from the zenity windows run by the updater don’t open at all. We should either find a way to fix them, or find a way to drop them.

The best long-term path might be to make the whole updater frontend a Perl/Gtk application, with embedded documentation when needed, but if we can find a shortcut for the shorter term, it would be good.

Parent Task: #5490 (closed)

Original created by @intrigeri on 6445 (Redmine)