Document the standalone discovery bootnode

[flcl42]

Changes

• Add a guide for running the standalone Bootnode with the published Docker image.
• Show how to connect a Nethermind node, query REST and JSON-RPC, and start Bootnode, Prometheus, and Grafana together.
• Explain the retained-node count and why it is not a live network-size estimate.

Testing and documentation

• Production docs build and the repository's Prettier check passed. Existing broken-anchor warnings are confined to unrelated pages.
• Validated the Compose merge and Prometheus configuration, then started the full stack: the scrape target was UP and Grafana loaded the provisioned dashboard.

Remarks

🤖 AI agent (Codex / GPT-6 Sol high) acting on behalf of the requester.