Make documentation for PingSource. (#2126)

* Document ping source.

* Squashed commit of the following:

commit 9a04cb6761
Author: Scott Nichols <snichols@vmware.com>
Date:   Thu Feb 27 07:32:03 2020 -0800

    Document ping source.

* typo on source.

* update image

* kail tabs

* by name default

docs/eventing/samples/ping-source/README.md

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+---
+title: "PingSource example"
+linkTitle: "PingSource"
+weight: 10
+type: "docs"
+---
+
+This example shows how to configure PingSource as an event source for
+functions.
+
+## Before you begin
+
+1. Set up [Knative Serving](../../../serving).
+1. Set up [Knative Eventing](../../../eventing).
+
+## Create a Knative Service
+
+To verify that `PingSource` is working, create a simple Knative
+Service that dumps incoming messages to its log.
+
+{{< tabs name="create-service" default="By YAML" >}}
+{{% tab name="By YAML" %}}
+Use following command to create the service from STDIN:
+
+```shell
+cat <<EOF | kubectl create -f -
+apiVersion: serving.knative.dev/v1
+kind: Service
+metadata:
+  name: event-display
+spec:
+  template:
+    spec:
+      containers:
+        - image: gcr.io/knative-releases/knative.dev/eventing-contrib/cmd/event_display
+EOF
+```
+{{< /tab >}}
+
+{{% tab name="By filename" %}}
+Use following command to create the service from the `service.yaml` file:
+
+```shell
+kubectl apply --filename service.yaml
+```
+{{< /tab >}}
+{{< /tabs >}}
+
+## Create a PingSource
+
+For each set of ping events that you want to request, create an Event
+Source in the same namespace as the destination.
+
+{{< tabs name="create-source" default="By YAML" >}}
+{{% tab name="By YAML" %}}
+Use following command to create the event source from STDIN:
+
+```shell
+cat <<EOF | kubectl create -f -
+apiVersion: sources.knative.dev/v1alpha1
+kind: PingSource
+metadata:
+  name: test-ping-source
+spec:
+  schedule: "*/2 * * * *"
+  data: '{"message": "Hello world!"}'
+  sink:
+    ref:
+      apiVersion: serving.knative.dev/v1
+      kind: Service
+      name: event-display
+EOF
+```
+{{< /tab >}}
+
+{{% tab name="By filename" %}}
+Use following command to create the event source from the `ping-source.yaml` file:
+
+```shell
+kubectl apply --filename ping-source.yaml
+```
+{{< /tab >}}
+{{< /tabs >}}
+
+
+## Verify
+
+Verify that the message was sent to the Knative eventing system by
+looking at message dumper logs.
+
+{{< tabs name="view-event" default="kubectl" >}}
+{{% tab name="kubectl" %}}
+
+Use following command to view the logs of the event-display service:
+
+```shell
+kubectl logs -l serving.knative.dev/service=event-display -c user-container --since=10m
+```
+
+{{< /tab >}}
+{{% tab name="kail" %}}
+
+You can also use [`kail`](https://github.com/boz/kail) instead of `kubectl logs`
+to tail the logs of the subscriber.
+
+```shell
+kail -l serving.knative.dev/service=event-display -c user-container --since=10m
+```
+
+{{< /tab >}}
+{{< /tabs >}}
+
+You should see log lines showing the request headers and body from the source:
+
+```
+☁️  cloudevents.Event
+Validation: valid
+Context Attributes,
+  specversion: 1.0
+  type: dev.knative.sources.ping
+  source: /apis/v1/namespaces/default/pingsources/test-ping-source
+  id: d8e761eb-30c7-49a3-a421-cd5895239f2d
+  time: 2019-12-04T14:24:00.000702251Z
+  datacontenttype: application/json
+Data,
+  {
+    "message": "Hello world!"
+  }
+```
+
+## Cleanup
+
+You can delete the PingSource instance by entering the following command:
+
+{{< tabs name="delete-source" default="By name" >}}
+{{% tab name="By name" %}}
+```shell
+kubectl delete pingsources.sources.knative.dev test-ping-source
+```
+{{< /tab >}}
+
+{{% tab name="By filename" %}}
+```shell
+kubectl delete --filename ping-source.yaml
+```
+{{< /tab >}}
+{{< /tabs >}}
+
+
+Similarly, you can delete the Service instance via:
+
+{{< tabs name="delete-service" default="By name" >}}
+{{% tab name="By name" %}}
+```shell
+kubectl delete service.serving.knative.dev event-display
+```
+{{< /tab >}}
+{{% tab name="By filename" %}}
+```shell
+kubectl delete --filename service.yaml
+```
+{{< /tab >}}
+
+{{< /tabs >}}

docs/eventing/samples/ping-source/ping-source.yaml

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+apiVersion: sources.knative.dev/v1alpha1
+kind: PingSource
+metadata:
+  name: test-ping-source
+spec:
+  schedule: "*/2 * * * *"
+  data: '{"message": "Hello world!"}'
+  sink:
+    ref:
+      apiVersion: serving.knative.dev/v1
+      kind: Service
+      name: event-display

docs/eventing/samples/ping-source/service.yaml

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+# This is a very simple Knative Service that writes the input request to its log.
+
+apiVersion: serving.knative.dev/v1
+kind: Service
+metadata:
+  name: event-display
+spec:
+  template:
+    spec:
+      containers:
+      - image: gcr.io/knative-releases/knative.dev/eventing-contrib/cmd/event_display