[docs] Document Django Channels WebSocket endpoints

[nemesifier]

Document Django Channels WebSocket API

Context: This repository implements Django Channels WebSocket endpoint(s) for batch operations:

• Batch operation: ws://<host>/ws/radius/batch/<batch_id>/

There is no formal documentation in the docs/ folder describing these endpoints or their message types.

Message Types

• batch_status_update (e.g. { "status": "completed" } )

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

• The WebSocket endpoint URLs and usage
• Message types with sample payloads
• Authentication and authorization requirements
• Purpose: Improve developer integration experience and maintainability

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