Update Connect documentation for 2.0

[mudge]

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 and rpi-connect off commands instead of using systemctl directly
• 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.

[nathan-contino]

Converting to draft since we're not ready to roll it out yet. But I'll get copy editing shortly!