Documentation around Pomerium/Istio/Grafana (#675)

* Added an example of how to protect Grafana with Pomerium inside of an Istio mesh
* Added relevant documentation links

docs/configuration/examples/kubernetes/istio/gateway.yml

@@ -0,0 +1,41 @@
+apiVersion: networking.istio.io/v1alpha3
+kind: Gateway
+metadata:
+  name: internal-gateway
+  namespace: istio-system
+spec:
+  selector:
+    istio: ingressgateway
+  servers:
+  - port:
+      number: 443
+      protocol: HTTPS
+      name: https-default
+    tls:
+      mode: SIMPLE
+      serverCertificate: "sds"
+      privateKey: "sds"
+      credentialName: internal-cert
+    hosts:
+    - *.yourcompany.com
+---
+apiVersion: cert-manager.io/v1alpha2
+kind: Certificate
+metadata:
+  name: internal-cert
+  namespace: istio-system
+spec:
+  secretName: internal-cert
+  issuerRef:
+    name: self-signed-issuer
+    kind: ClusterIssuer
+  commonName: *.yourcompany.com
+  dnsNames:
+    - *.yourcompany.com
+---
+apiVersion: cert-manager.io/v1alpha2
+kind: ClusterIssuer
+metadata:
+  name: self-signed-issuer
+spec:
+  selfSigned: {}