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.

As neither rpi-connect 1.4.0 nor rpi-connect-lite 1.4.0 clean up their
systemd symlinks or stop their services on uninstall, it is important
that users upgrade from rpi-connect 1.4.0 to rpi-connect 2.0.0 or they
upgrade from rpi-connect-lite 1.4.0 to rpi-connect-lite 2.0.0.

If they attempt to switch packages during the upgrade (i.e. go from
rpi-connect 1.4.0 to rpi-connect-lite 2.0.0 or from rpi-connect-lite
1.4.0 to rpi-connect 2.0.0), the necessary migration script will not run
leaving them in an inconsistent state until they either purge their old
package or upgrade it.

Document this issue by encouraging users to upgrade their existing
package in place before any attempt to switch between rpi-connect and
rpi-connect-lite.