docs: add federated gateway overview (fixes #4454)

docs/federated-gateway-overview.md

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+# Federated Gateway — Overview
+
+## What is a Federated Gateway?
+A **Federated Gateway** is an architectural pattern that allows multiple API gateway instances (often deployed in different data centers or cloud regions) to operate in a coordinated manner. Federation enables scaling, locality of traffic, and multi-tenant scenarios while preserving a central control plane for API management.
+
+## Key concepts
+- **Control plane vs Data plane** — The control plane manages API lifecycle, policies, and configuration; the data plane (gateways) handles runtime traffic enforcement.
+- **Gateway federation** — Multiple gateway instances share configuration and policies from a central management plane.
+- **Tenant and domain isolation** — Gateways can serve different tenant domains with isolated routing and policies.
+
+## Typical use cases
+- Multi-region or multi-DC deployments for low latency.
+- Regulatory or data residency requirements that require traffic to stay within geographical boundaries.
+- High availability and failover configurations.
+
+## Deployment considerations
+- **Configuration synchronization:** Use a reliable sync mechanism (push from control plane or pull by gateways).
+- **Policy compatibility:** Ensure gateways support the same policy set and runtime features.
+- **Monitoring and logging:** Centralize telemetry from all gateways for unified observability.
+
+## Related topics
+- API Manager control plane and gateway deployment
+- Gateway clusters and scaling
+- Security and tenant isolation
+
+For more detailed examples and configuration steps, see the Gateway deployment section in this documentation.