[codex] design network device backups

[jmgilman]

Summary

• Add a dedicated network-device backup design page for RouterOS devices.
• Document the intended platform-cluster placement, Oxidized collection flow, SOPS-encrypted Git writer, private secrets repo boundary, and restore expectations.
• Wire the new page from the docs landing page, architecture overview, and repository README.

Validation

• npm ci
• npm run typecheck
• npm run build

Note: npm ci reported 18 high-severity vulnerabilities in existing npm dependencies. No dependency changes were made in this PR.