docs(cni): add resiliency guidance

[roshnicdave]

Summary

• add a CNI resiliency planning page
• document device diversity, automatic failover, v1 LACP and v2 ECMP guidance, and backup connectivity
• clarify that Dashboard status does not validate end-to-end failover

The page intentionally excludes unshipped resiliency posture, tier, maintenance-zone, and Dashboard configuration behavior.

Validation

• pnpm run check
• pnpm run build

[cloudflare-docs-bot]

Review

✅ No issues found in commit 8bdf27b.

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