doc: Fallback Sink (numaproj#1691)

Signed-off-by: Yashash H L <[email protected]>
Signed-off-by: Vigith Maurice <[email protected]>
Co-authored-by: Vigith Maurice <[email protected]>

Author: yhl25

docs/user-guide/sinks/fallback.md

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+# Fallback Sink
+
+A `Fallback` Sink functions as a `Dead Letter Queue (DLQ)` Sink and can be configured to serve as a backup when the primary sink is down, 
+unavailable, or under maintenance. This is particularly useful when multiple sinks are in a pipeline; if a sink fails, the resulting 
+back-pressure will back-propagate and stop the source vertex from reading more data. A `Fallback` Sink can beset up to prevent this from happening. 
+This backup sink stores data while the primary sink is offline. The stored data can be replayed once the primary sink is back online.
+
+Note: The `fallback` field is optional. 
+
+Users are required to return a fallback response from the [user-defined sink](https://numaflow.numaproj.io/user-guide/sinks/user-defined-sinks/) when the primary sink fails; only
+then the messages will be directed to the fallback sink. 
+
+Example of a fallback response in a user-defined sink: [here](https://github.com/numaproj/numaflow-go/blob/main/pkg/sinker/examples/fallback/main.go)
+
+## CAVEATs
+The `fallback` field can only be utilized when the primary sink is a `User Defined Sink.`
+
+
+## Example
+
+### Builtin Kafka
+An example using builtin kafka as fallback sink:
+
+```yaml
+    - name: out
+      sink:
+        udsink:
+          container:
+            image: my-sink:latest
+        fallback:
+          kafka:
+            brokers:
+              - my-broker1:19700
+              - my-broker2:19700
+            topic: my-topic
+```
+### UD Sink
+An example using custom user-defined sink as fallback sink.
+
+User Defined Sink as a fallback sink:
+```yaml
+    - name: out
+      sink:
+        udsink:
+          container:
+            image: my-sink:latest
+        fallback:
+          udsink:
+            container:
+              image: my-sink:latest
+```

examples/14-simple-fallback-pipeline.yaml

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+apiVersion: numaflow.numaproj.io/v1alpha1
+kind: Pipeline
+metadata:
+  name: simple-fallback
+spec:
+  vertices:
+    - name: in
+      source:
+        # A self data generating source
+        generator:
+          rpu: 5
+          duration: 1s
+    - name: cat
+      scale:
+        min: 1
+      udf:
+        builtin:
+          name: cat
+    - name: output
+      scale:
+        min: 1
+      sink:
+        udsink:
+          container:
+            image: quay.io/numaio/numaflow-go/fb-sink-log:v0.7.0-rc2
+            imagePullPolicy: Always
+        fallback:
+          udsink:
+            container:
+              image: quay.io/numaio/numaflow-go/fb-sink-log:v0.7.0-rc2
+  edges:
+    - from: in
+      to: cat
+    - from: cat
+      to: output