Docs: add a practical single-node monitoring guide for Arc nodes on Ubuntu #23

Hi team,

I’ve successfully deployed and monitored a single Arc node on Ubuntu x86_64 bare-metal using the Arc execution and consensus metrics endpoints together with Prometheus and Grafana.

While the repository already contains monitoring assets, the current setup appears to be oriented more toward localdev / multi-node environments. I think it would be helpful to add a practical single-node monitoring guide for operators running one execution layer and one consensus layer on the same host.

The guide I’d like to contribute would cover:

• Prometheus scrape targets for a single node
• Grafana setup reusing the official Arc dashboards
• local-only binding for monitoring services
• SSH tunnel access
• troubleshooting notes for common issues, including:
  • Prometheus permission errors on /prometheus/queries.active
  • confusion around 127.0.0.1 bindings vs public IP access

If this sounds useful, I’d be happy to open a docs PR.