docs: [torrust#64] add Grafana tracker dashboard screenshot

Author: josecelano

README.md

@@ -11,11 +11,17 @@ It's also used to track issues in production.
 - HTTP Tracker: <https://tracker.torrust-demo.com/announce>
 - UDP Tracker: udp://tracker.torrust-demo.com:6969/announce
 
+### Monitoring
+
+The demo includes monitoring with Grafana dashboards for the tracker to track performance metrics, peer connections, and system health.
+
+![Grafana Dashboard](docs/media/torrust-tracker-grafana-dashboard.png)
+
 ## Documentation
 
 - [Setup](docs/setup.md)
 - [Deployment](docs/deployment.md)
 - [Firewall](docs/firewall.md)
 - [Sample Commands](docs/sample_commands.md)
 - [Rollbacks](docs/rollbacks.md)
-- [Backups](docs/backups.md)
+- [Backups](docs/backups.md)