Update Connect documentation for 2.0

Specifically:

* Update the documentation for the new wfpanel plugin which replaces the
  built-in user interface
* Update all screenshots with the latest Connect website
* Prefer the new rpi-connect on, rpi-connect off, and rpi-connect
  restart commands instead of using systemctl directly
* Remove instructions to manually restart after upgrade as this will now
  be done automatically
* Promote the troubleshooting tips posted to the forums to the official
  documentation

Note that the new user interface is a plugin for wfpanel which is
separate from the "system tray" so switch to referring to the new icon
as being "in the menu bar" to avoid any ambiguity.

Author: mudge