Add docs for infrastructure field (#3455)

* Add docs for infrastructure field

* correct link markdown

* pretty formatting

Author: craigbox

mkdocs.yml

@@ -104,6 +104,7 @@ nav:
       - gRPC Routing: guides/grpc-routing.md
       - Migrating from Ingress: guides/migrating-from-ingress.md
       - Backend Protocol Selection: guides/backend-protocol.md
+      - Infrastructure attributes: guides/infrastructure.md
     - API Design: guides/api-design.md
     - CRD Management: guides/crd-management.md
     - Implementer's Guide: guides/implementers.md

site-src/guides/infrastructure.md

@@ -0,0 +1,45 @@
+# Gateway infrastructure labels and annotations
+
+??? success "Standard Channel since v1.2.0"
+
+    The `infrastructure` field is GA and has been part of the Standard Channel since
+    `v1.2.0`. For more information on release channels, refer to our [versioning
+    guide](/concepts/versioning).
+
+Gateway API implementations are responsible for creating the backing
+infrastructure needed to make each Gateway work. For example, implementations
+running in a Kubernetes cluster often create [Services][service] and
+[Deployments][deployment], while cloud-based implementations may create cloud
+load balancer resources. In many cases, it can be helpful to be able to
+propagate labels or annotations to these generated resources.
+
+
+The [`infrastructure` field][infrastructure] on a Gateway allows you to specify
+labels and annotations for the infrastructure created by the Gateway API controller.
+For example, if your Gateway infrastructure is running in-cluster, you can specify
+both Linkerd and Istio injection using the following Gateway configuration, making
+it simpler for the infrastructure to be incorporated into whichever service mesh
+you've installed.
+
+```yaml
+apiVersion: gateway.networking.k8s.io/v1
+kind: Gateway
+metadata:
+  name: meshed-gateway
+  namespace: incoming
+spec:
+  gatewayClassName: meshed-gateway-class
+  listeners:
+  - name: http-listener
+    protocol: HTTP
+    port: 80
+  infrastructure:
+    labels:
+      istio-injection: enabled
+    annotations:
+      linkerd.io/inject: enabled
+```
+
+[infrastructure]: /reference/spec/#gateway.networking.k8s.io/v1.GatewayInfrastructure
+[service]: https://kubernetes.io/docs/concepts/services-networking/service/
+[deployment]: https://kubernetes.io/docs/concepts/workloads/controllers/deployment/