docs: mainnet operational runbook (admin custody, launch sequence, incident escalation) - #208

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, peaceiris/actions-mdbook@v2. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/