Kafka gateway: Kafka-to-Iggy migration guide — dual-write, cutover, decommission

[ryerraguntla]

Part of #3560.

Summary

Document the four-step migration path for teams moving from Kafka to Iggy via the gateway.

Steps

1. Deploy Iggy + bridge (sidecar)
2. Dual-write to Kafka and Iggy
3. Consumer cutover (offset mapping / consumer group strategy)
4. Decommission Kafka

Depends on

• Kafka gateway: Kafka gateway Phase 1 — docker-compose, E2E tests, and quick-start docs #3539 — Phase 1 documented
• Kafka gateway: Kafka gateway Phase 2 — E2E tests with consumer groups #3544 — Phase 2 for --group cutover section (can draft P1 section earlier)

Scope

• Architecture diagram (standalone sidecar)
• Dual-write patterns (app-level vs mirror)
• Offset / consumer group migration considerations
• Rollback: stop bridge → clients fail gracefully
• Operational checklist: monitoring, latency, scaling bridge vs Iggy

Acceptance criteria

• Published under gateways/kafka/docs/ or apache.iggy.org
• Linked from discussion feature - Order of Implementation discussion of kafka to iggy bridge #3253

References

• Rollout plan: feature - Order of Implementation discussion of kafka to iggy bridge #3253 (comment)