docs(k8s): add HTTPRoutes support (#6854)

Signed-off-by: germangarces <german.garces@flagsmith.com>

Author: germangarces

docs/docs/deployment-self-hosting/hosting-guides/kubernetes-openshift.md

@@ -139,6 +139,88 @@ Run `minikube ip`. Set this IP and `flagsmith.local` in your `/etc/hosts`, for e
 
 Then access `http://flagsmith.local` in a browser.
 
+### Using Gateway API (alternative to Ingress)
+
+The chart supports [Gateway API](https://gateway-api.sigs.k8s.io/) HTTPRoutes as an alternative to Ingress resources.
+
+#### Prerequisites
+
+Install these independently before enabling HTTPRoutes:
+
+1. **Gateway API CRDs**:
+   ```bash
+   kubectl apply -f https://github.com/kubernetes-sigs/gateway-api/releases/latest/download/standard-install.yaml
+   ```
+2. **A Gateway controller** (e.g., [Envoy Gateway](https://gateway.envoyproxy.io/), Cilium, Istio)
+3. **A GatewayClass and Gateway resource** in your cluster. For example, using Envoy Gateway:
+
+   ```yaml
+   apiVersion: gateway.networking.k8s.io/v1
+   kind: GatewayClass
+   metadata:
+     name: eg
+   spec:
+     controllerName: gateway.envoyproxy.io/gatewayclass-controller
+   ---
+   apiVersion: gateway.networking.k8s.io/v1
+   kind: Gateway
+   metadata:
+     name: my-gateway
+     namespace: gateway-ns
+   spec:
+     gatewayClassName: eg
+     listeners:
+       - name: http
+         port: 80
+         protocol: HTTP
+         allowedRoutes:
+           namespaces:
+             from: All
+   ```
+
+#### Configuration
+
+The chart only manages HTTPRoute resources. The frontend defaults to routing `/` to the frontend service. The API and SSE routes have no defaults — you must provide rules explicitly.
+
+A typical configuration with separate frontend and API routes:
+
+```yaml
+gateway:
+  frontend:
+    enabled: true
+    parentRefs:
+      - name: my-gateway
+        namespace: gateway-ns
+  api:
+    enabled: true
+    parentRefs:
+      - name: my-gateway
+        namespace: gateway-ns
+    rules:
+      - matches:
+          - path:
+              type: PathPrefix
+              value: /api/
+          - path:
+              type: PathPrefix
+              value: /health/
+          - path:
+              type: PathPrefix
+              value: /admin/
+          - path:
+              type: PathPrefix
+              value: /static/admin/
+        backendRefs:
+          - name: flagsmith-api
+            port: 8000
+
+frontend:
+  extraEnv:
+    FLAGSMITH_API_URL: 'https://flagsmith.[MYDOMAIN]/api/v1/'
+```
+
+See the [`values.yaml`](https://github.com/Flagsmith/flagsmith-charts/blob/main/charts/flagsmith/values.yaml) for all available options.
+
 ### Provided Database configuration
 
 By default, the chart creates its own PostgreSQL server within the cluster, referencing [https://github.com/helm/charts/tree/master/stable/postgresql](https://github.com/helm/charts/tree/master/stable/postgresql) for the service.
@@ -522,4 +604,4 @@ To build chart package run:
 
 ```bash
 helm package ./flagsmith
-``` 
+```