docs: add SessionStateListener reference page

[Copilot]

Adds a missing reference page for quickfix.SessionStateListener — a default-method interface for in-process session lifecycle hooks (connect, logon, heartbeat, resend, sequence reset, PossDup discard, etc.) that complements the Application interface without requiring coupling to business logic.

Changes

• docs/session-state-listener.md — new page covering:

  • Full interface definition (all 13 callbacks, correct signatures with SessionID params, including onConnectException, onResendRequestSent, onSequenceResetReceived, onResendRequestSatisfied, and onPossDupMessageDiscarded)
  • Registration pattern via Session.lookupSession() in onCreate
  • Complete example implementation
  • Multiple-listener registration
  • Application vs SessionStateListener comparison table
  • Cross-reference to JMX notifications

• sidebars.ts — session-state-listener inserted into the Developer Guide section between developer-docs and repeating-groups

// Register one or more listeners per session
Session session = Session.lookupSession(sessionId);
session.addStateListener(new MetricsListener());
session.addStateListener(new AlertingListener());