docs: add channel-based WebSocket subscription docs

Document the new channels system that lets clients subscribe to specific
data types (ev, arbitrage, middles, low_hold, odds) to reduce payload
size. Adds documentation for opportunities_snapshot, snapshot_complete,
middles:detected/expired, and low_hold:detected/expired message types.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Mlaz-code