docs: Add missing YAML file and fix ingress instructions

The instructions for the Ingress referred to a missing YAML file, so
this patch adds the file and updates the docs.

Author: joaquimrocha

docs/installation/in-cluster.md

@@ -28,7 +28,7 @@ Assuming your URL is `headlamp.mydeployment.io`, getting the sample ingress
 file and changing the URL can quickly be done by:
 
 ```bash
-curl -s https://raw.githubusercontent.com/kinvolk/headlamp/jrocha/wip/delme/kubernetes-headlamp-ingress-sample.yaml | sed -e s/__URL__/headlamp.mydeployment.io/ > headlamp-ingress.yaml
+curl -s https://raw.githubusercontent.com/kinvolk/headlamp/master/kubernetes-headlamp-ingress-sample.yaml | sed -e s/__URL__/headlamp.mydeployment.io/ > headlamp-ingress.yaml
 ```
 
 and with that, you'll have a configured ingress file, so verify it and apply it:

kubernetes-headlamp-ingress-sample.yaml

@@ -0,0 +1,24 @@
+kind: Ingress
+apiVersion: extensions/v1beta1
+metadata:
+  name: headlamp
+  namespace: kube-system
+  annotations:
+    kubernetes.io/tls-acme: "true"
+    cert-manager.io/cluster-issuer: "letsencrypt-production"
+    kubernetes.io/ingress.class: contour
+spec:
+  tls:
+  - secretName: headlamp
+    hosts:
+    - __URL__
+  rules:
+  -
+    host: __URL__
+    http:
+      paths:
+      -
+        path: /
+        backend:
+          serviceName: headlamp
+          servicePort: 80