docs: add Grafana dashboard setup guide and provisioning JSON for Prometheus users

[Vaishnav88sk]

The existing docs/DASHBOARDS.md only covers Kibana (Elastic) dashboards. Users running the Prometheus + Grafana stack - a common open-source observability setup - have no documentation showing how to visualise Jenkins metrics in Grafana.

This PR adds:

• docs/grafana-dashboard-setup.md - end-to-end guide covering:

  • OTel Collector config with Prometheus exporter and resource_to_telemetry_conversion (required for label filtering)
  • Prometheus scrape config
  • Dashboard import via Grafana UI and via provisioning
  • JCasC config to link Jenkins build pages back to Grafana using customObservabilityBackend
  • Troubleshooting section for missing metrics and the ci_pipeline_run_duration_milliseconds allow-list config

• docs/grafana/jenkins-overview.json - provisioning-ready Grafana
  dashboard JSON with panels for:

  • Build count, success rate, p50/p95 build duration (stat panels)
  • Build results over time by status (SUCCESS / FAILURE / ABORTED)
  • Queue wait time p50/p95
  • Executor busy vs idle
  • JVM heap memory and HTTP request duration
  • service_name template variable for multi-instance filtering

• docs/DASHBOARDS.md - added link to the new Grafana guide

Relates to the gap identified in docs/monitoring-metrics.md, which lists all available metrics but provides no Grafana visualisation guidance.

Testing done

Documentation and dashboard JSON only - no code changes.

• All PromQL expressions in jenkins-overview.json verified against metrics emitted by the plugin (ci_pipeline_run_duration_milliseconds, ci_pipeline_run_queue_duration_milliseconds, jenkins_executor_count, runtime_jvm_memory_usage, http_server_request_duration_seconds)

• Dashboard JSON imported into Grafana 10.x locally against a Prometheus instance scraping the OTel Collector - all panels render without errors

• Provisioning config tested: copied JSON to /etc/grafana/provisioning/dashboards/ and confirmed dashboard appears automatically after Grafana restart

• All cross-links in the markdown verified to point to existing files in the repo (setup-and-configuration.md, monitoring-metrics.md, DASHBOARDS.md)

• OTel Collector config in the guide tested locally with otelcol-contrib validate --config otel-collector-config.yaml

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

Fixes #1276