docs: include exchange in WebSocket examples

[999axel999]

Summary

• Add the required exchange field to WebSocket subscribe examples.
• Include the matching method and args fields in unsubscribe examples so they match server subscription lookup.
• Cover the overview, order book, batch order book, and trades WebSocket docs.

Tests

• git diff --check
• Select-String verification for "exchange": "polymarket" in updated examples
• Checked core/src/server/ws-handler.ts requires id, action, and exchange

Prepared with local automation assistance and reviewed before submission.