Document a two-instance collaboration debugging session

[Ayush-Kumar0207]

Problem

The benchmark proves that two Socket.IO instances can share Redis-backed room state, but contributors do not yet have a short debugging walkthrough for running that topology and observing reconnect behavior.

What good looks like

• document the smallest reproducible two-server setup using the existing Docker and Redis tooling;
• include the expected join, edit propagation, reconnect, and permission-rejection signals;
• include cleanup commands and troubleshooting for occupied ports, unavailable Redis, and stale sessions;
• link to the relevant server modules instead of copying implementation details;
• validate every command on a clean local checkout;
• link the walkthrough from CONTRIBUTING.md and the collaboration benchmark report.

Likely files

• a new guide under docs/
• CONTRIBUTING.md
• docs/COLLABORATION_BENCHMARKS.md

Please comment with the operating-system and container setup you plan to validate before starting.