docs: add single-host monitoring guide for Arc EL + CL

[Bela1401]

Summary

This PR adds a practical monitoring guide for operators running a single Arc
execution layer + consensus layer on one Ubuntu host.

It documents:

• Prometheus scrape targets for EL and CL metrics
• a minimal Grafana + Prometheus setup
• dashboard provisioning
• local access via SSH tunneling
• common troubleshooting notes observed during real-world setup

Scope

This PR intentionally stays focused on a single-host operator setup and does not
attempt to cover multi-node or production observability architectures.

Related docs change

This PR also adds a reference from docs/running-an-arc-node.md to the new
docs/monitoring.md guide.

Notes

I kept the scope aligned with the feedback in the discussion:

• placement under docs/monitoring.md
• focused on one EL + one CL on one host
• reuses the existing monitoring dashboards already present in the repository

[Bela1401]

Opened this PR with the scope kept intentionally narrow: one execution layer + one consensus layer on one host, with Prometheus and Grafana for operator monitoring.

I also added the link from docs/running-an-arc-node.md to the new monitoring guide.

[Bela1401]

Validation screenshot from a live single-host setup is attached below.