[documentation] Add Render production deployment runbook #39

Context

render.yaml defines gateway, API, and Postgres but README only covers local docker-compose and cargo run.

Problem

Maintainers lack step-by-step production deploy instructions including secrets, webhook URL, and contract id setup.

Proposed scope

• Add docs/deployment/render.md runbook
• Cover Blueprint deploy, env vars, GitHub webhook URL, and health verification
• Include rollback and pause program operator steps

Acceptance criteria

• Runbook covers full Render deploy from empty project
• Lists all required secrets with rotation notes
• Verifies health, metrics, and sample admin API call
• Linked from README documentation section

References

• render.yaml
• .env.example
• docs/security-checklist.md
• Dockerfile.gateway, Dockerfile.api