docs(cni): clarify maintenance coordination

[roshnicdave]

Summary

• add a CNI Maintenance page and move existing timing and scheduling guidance into it
• document routine, non-routine, and emergency maintenance behavior for resilient deployments
• retain Operational guidance for failover responsibility and troubleshooting
• leave existing maintenance-alert configuration and eligibility wording unchanged

Validation

• pnpm run check
• pnpm run build

[cloudflare-docs-bot]

Review

✅ No issues found in commit b51a3ea.

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