Add documentation for the different knative services and deployments (#1467)

that are running on a kubernetes cluster after installing knative 0.6.0.

Author: trshafer

docs/serving/knative-kubernetes-services.md

@@ -0,0 +1,64 @@
+---
+title: "Knative Kubernetes Services"
+#linkTitle: "OPTIONAL_ALTERNATE_NAV_TITLE"
+weight: 9
+type: "docs"
+---
+
+This document describes what is running when running knative serving.
+
+After applying the serving yaml, this will install a few knative services and deployments on your kubernetes cluster. This document provides an overview of the deployments and the motivations for each one.
+
+```sh
+$ kubectl get services -n knative-serving
+
+NAME                TYPE        CLUSTER-IP       EXTERNAL-IP   PORT(S)                  AGE
+activator-service   ClusterIP   10.96.61.11      <none>        80/TCP,81/TCP,9090/TCP   1h
+autoscaler          ClusterIP   10.104.217.223   <none>        8080/TCP,9090/TCP        1h
+controller          ClusterIP   10.101.39.220    <none>        9090/TCP                 1h
+webhook             ClusterIP   10.107.144.50    <none>        443/TCP                  1h
+```
+
+```sh
+$ kubectl get deployments -n knative-serving
+
+NAME                     DESIRED   CURRENT   UP-TO-DATE   AVAILABLE   AGE
+activator                1         1         1            1           1h
+autoscaler               1         1         1            1           1h
+controller               1         1         1            1           1h
+networking-certmanager   1         1         1            1           1h
+networking-istio         1         1         1            1           1h
+webhook                  1         1         1            1           1h
+```
+
+## Service: activator
+
+The responsibilities of the activator are:
+* Receiving & buffering requests for inactive Revisions.
+* Reporting metrics to the autoscaler.
+* Retrying requests to a Revision after the autoscaler scales such Revision based on the reported metrics.
+
+## Service: autoscaler
+
+The autoscaler receives request metrics and adjusts the number of pods required to handle the load of traffic. 
+
+## Service: controller
+
+The controller service reconciles all the public knative objects and autoscaling CRDs. When a user applies a knative service to the kubernetes api, this creates the config and route. It will convert config into revisions. It will convert Revision into Deployment and KPA.
+
+## Service: webhook
+
+The webhook intercepts all kubernetes api calls, all crd insertions and updates. It does two things:
+
+1. Set default values
+2. Rejects inconsitent and invalid objects.
+
+It validates and mutates k8s api calls.
+
+## Deployment: networking-certmanager
+
+The certmanager reconciles cluster ingress into cert manager objects.
+
+## Deployment: networking-istio
+
+This reconciles cluster ingress into a virtual service.