Add a Grafana/Prometheus dashboard template for Woodpecker

[tinswzy]

In issue #16, we added detailed Prometheus metrics for all components of Woodpecker.

To enhance observability and make it easier to monitor the system, we now need a reference Grafana dashboard template. This template should cover key metrics such as WAL write throughput, read latency, segment state tracking, etc., and serve as a starting point for users or operators.
Goals:

• Provide an importable Grafana dashboard JSON template;
• Include visualizations for core Woodpecker metrics;
• Upload the template file to the user documentation.

[tinswzy]

When adding the service mode, the grafana Monitoring dashboard template has also been added