[docs] Document Django Channels WebSocket endpoints

[nemesifier]

Document Django Channels WebSocket API

Context: This repository implements Django Channels WebSocket endpoints for device commands and geo location data. However, no formal documentation exists in the docs/ folder about these endpoints or their message types.

Example endpoints

• Device command: ws://<host>/ws/controller/device/<device_id>/command
• Geo location (broadcast): ws://<host>/ws/controller/location/<location_id>

Message Types

(Some message types are implemented via custom consumers)

Suggestion:
Please create documentation in docs/user/websocket-api.rst (or a similar location) describing:

• The connection URLs for all WebSocket endpoints provided by the project.
• The message types, including sample payloads.
• Authentication/authorization requirements for connecting.
• Purpose: Helps developers build custom integrations and improves maintainability.

This is modeled after the suggestion in openwisp-firmware-upgrader.