Add WebSocket connection documentation

[rbren]

This PR adds documentation for connecting to the OpenHands WebSocket API:

• Adds a user guide for connecting to the WebSocket API
• Updates the sidebar to include the new documentation

The documentation includes:

• Connection parameters and protocol details
• Event types and message formats
• Code examples in JavaScript
• Examples using websocat for testing

---

To run this PR locally, use the following command:

docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:623ca84-nikolaik   --name openhands-app-623ca84   docker.all-hands.dev/all-hands-ai/openhands:623ca84

[rbren]

@OpenHands make it clear that if your connecting via OpenHands Cloud, you should provision an API key, and then pass the header Authorization: Bearer $API_KEY to websocat or whatever other websocket client

Link to https://docs.all-hands.dev/modules/usage/cloud/cloud-api for getting an API key

Also add a link to that page (inside the docs/ folder) linking to this page and explaining that this is how you connect to the websocket

[mamoodi]

Ack on this. Is the information accurate? Have you done an initial review?

[rbren]

I've read through it, but haven't run the code samples

@OpenHands replace the placeholder http://your-openhands-server with http://localhost:3000

[openhands-ai]

I'm on it! rbren can track my progress at all-hands.dev

[openhands-ai]

I successfully addressed the PR comment by replacing all instances of the placeholder URL in the WebSocket documentation.

Here's what I did:

1. Located all instances of "http://your-openhands-server" and "ws://your-openhands-server" in the WebSocket documentation file (docs/modules/usage/how-to/websocket-connection.md)
2. Replaced them with "http://localhost:3000" and "ws://localhost:3000" respectively
3. Committed the changes with the message "Replace placeholder server URL with localhost:3000"
4. Pushed the changes to the add-websocket-documentation branch

The PR #8404 has been updated with these changes. All code examples in the WebSocket documentation now use localhost:3000 as the server URL instead of the placeholder.