Kafka ResetOffset Examples (#4012)

* Kafka ResetOffset Examples

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* Use a better Kafka Channel install link

docs/eventing/samples/kafka/resetoffset/README.md

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+---
+title: "Apache Kafka ResetOffset Example"
+linkTitle: "ResetOffset Example"
+weight: 20
+type: "docs"
+---
+
+Kafka backed Channels are potentially different from other implementations in
+that they provide temporal persistence of events. This capability allows for
+certain advanced use cases to be supported, including the ability to "replay"
+prior events via repositioning the current location in the event-stream.  This
+capability can be extremely useful when attempting to recover from unexpected
+Subscriber downtime.
+
+The
+[ResetOffset CRD](https://github.com/knative-sandbox/eventing-kafka/tree/main/config/command/resetoffset)
+exposes the ability to manipulate the location of the ConsumerGroup Offsets in
+the event stream of a given Knative Subscription. Without the ResetOffset CRD
+you would be responsible for manually stopping ConsumerGroups and manipulating
+Offsets.
+
+!!! note
+    Repositioning the ConsumerGroup Offsets will impact the event ordering and
+    is intended for failure recovery scenarios. This capability needs to be used
+    with caution only after reviewing the CRD documentation linked above.
+
+!!! note
+    ResetOffsets are currently only supported by the Distributed KafkaChannel
+    implementation. It is expected that the Consolidated KafkaChannel
+    implementation will support them in the near future.
+
+## Prerequisites
+
+- A Kubernetes cluster with a Knative
+  [Kafka Channel](https://knative.dev/docs/eventing/channels/channels-crds/)
+  installed.
+- A valid KafkaChannel and Subscription exist.
+
+## Examples
+
+The following examples assume a Kubernetes Namespace called `my-namespace`
+containing a KafkaChannel Subscription named `my-subscription` and should be
+updated according to your use case.
+
+### Repositioning the Offsets to the oldest available event
+
+The following ResetOffset instance will move the offsets back to the oldest
+available event in the Kafka Topic retention window.
+
+```yaml
+apiVersion: kafka.eventing.knative.dev/v1alpha1
+kind: ResetOffset
+metadata:
+  name: my-resetoffset
+  namespace: my-namespace
+spec:
+  offset:
+    time: earliest
+  ref:
+    apiVersion: messaging.knative.dev/v1
+    kind: Subscription
+    namespace: my-namespace
+    name: my-subscription
+```
+
+### Repositioning the Offsets to a specific point in the event stream
+
+The following ResetOffset instance will move the offsets back to the specified
+point in the Kafka Topic retention window. You will need to set an appropriate
+[RFC3339](https://datatracker.ietf.org/doc/html/rfc3339) timestamp relative to
+your use case.
+
+```yaml
+apiVersion: kafka.eventing.knative.dev/v1alpha1
+kind: ResetOffset
+metadata:
+  name: my-resetoffset
+  namespace: my-namespace
+spec:
+  offset:
+    time: "2021-06-17T17:30:00Z"
+  ref:
+    apiVersion: messaging.knative.dev/v1
+    kind: Subscription
+    namespace: my-namespace
+    name: my-subscription
+```
+
+!!! note
+    The ResetOffset CRD is a single use operation or "command", and should be
+    deleted from the cluster once it has completed execution.