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

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

- #3539 — Phase 1 documented
- #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 #3253

## References

- Rollout plan: https://github.com/apache/iggy/discussions/3253#discussioncomment-17181030

Start by reading #3539, #3544, and the rollout plan in discussion #3253, then inspect the existing material under gateways/kafka/docs/. Draft the four-step migration guide covering the listed architecture, dual-write, offset and consumer-group, rollback, and operational topics. Done means it is published under gateways/kafka/docs/ or apache.iggy.org and linked from #3253.

Written by the indexing model from the issue text.