docs: add configuring ingress controller nginx example

docs/administration/configuration/configuring-ingress-controller/index.md

@@ -0,0 +1,51 @@
+# Configuring Ingress Controller
+
+By default, REANA uses `traefik` as an ingress controller. However, it is possible to replace the `traefik` ingress with an `nginx` ingress.
+
+Currently, this change is only possible manually, as it is not yet fully supported by the REANA helm chart.
+
+The following code snipped shows a possible `nginx` ingress manifest:
+
+```yaml
+apiVersion: networking.k8s.io/v1
+kind: Ingress
+metadata:
+  annotations:
+    ingress.kubernetes.io/ssl-redirect: "true"
+    kubernetes.io/ingress.class: nginx
+  name: <helm-release-name>-ingress
+  namespace: <reana-namespace>
+spec:
+  rules:
+  - http:
+      paths:
+      - backend:
+          service:
+            name: <helm-release-name>-server
+            port:
+              number: 80
+        path: /api
+        pathType: Prefix
+      - backend:
+          service:
+            name: <helm-release-name>-server
+            port:
+              number: 80
+        path: /oauth
+        pathType: Prefix
+      - backend:
+          service:
+            name: <helm-release-name>-ui
+            port:
+              number: 80
+        path: /
+        pathType: Prefix
+  tls:
+  - secretName: <helm-release-name>-tls-secret
+status:
+  loadBalancer:
+    ingress:
+    - ip: <lb-ip>
+```
+
+which can be deployed manually via: `kubectl apply -f <ingress-yaml-filename>.yaml`.