docs: [#246] Add extension tasks for Grafana health checks and auto-provisioning

- Add 4 extension tasks for issue #246 (Grafana integration)
- Task 1: Add Prometheus health check to docker-compose
- Task 2: Add Grafana health check with optional Prometheus dependency
- Task 3: Auto-configure Prometheus datasource via provisioning
- Task 4: Preload dashboards (stats.json and metrics.json from torrust-demo)
- Include complete implementation details with code examples
- Add Ansible playbook design (deploy-grafana-provisioning.yml)
- Add comprehensive manual testing guide (400+ lines)
- Document Prometheus job mapping (tracker_stats, tracker_metrics)
- Use actual dashboard files from torrust-demo repository
- Add Grafonnet to project dictionary for spell checking

Total effort: 10-16 hours across 4 independently trackable tasks

Author: josecelano