Document Nightscout WebSocket toggle

[bjorkert]

Summary

Adds a short WebSocket subsection under Setup → Setup Nightscout describing the new Enable WebSocket toggle:

• In the foreground, LoopFollow keeps a live connection to Nightscout so updates arrive within seconds — close to real-time.
• When the app moves to the background, the connection is dropped and LoopFollow falls back to its normal scheduled polling to save battery. The connection is re-established when the app returns to the foreground.
• The status row (Connecting… / Connected / Disconnected / Error) is documented.
• Falls back to normal polling immediately if the connection drops while in the foreground.

Companion PR

Implementation: loopandlearn/LoopFollow#606