docs: add Home Assistant custom component integration guide for MQTT diagnostics

- Add comprehensive section on integrating MqttDiagnosticsCollector into HA custom components
- Recommend saving diagnostics to Home Assistant config directory vs. alternatives
- Explain trade-offs: config dir (recommended) vs. data store, logs, and separate files
- Include integration pattern with HA diagnostics protocol
- Provide minimal working example of HA component with integrated diagnostics
- Add periodic export schedule pattern for production components

Author: eman