Update the Kubernetes deployment documentation with GKE environment setup

From Slack chat:

For a GKE Ingress make these modifications:

• templates/openslice-ingress.yaml

-          - pathType: ImplementationSpecific
+          - pathType: Prefix

• openslice/values.yaml

ingress:
-  ingressClassName: nginx
-  annotations: {}
+  annotations:
+    kubernetes.io/ingress.class: gce
+    kubernetes.io/ingress.global-static-ip-name: web-static-ip