Documentation for Indexed completion mode

Signed-off-by: Aldo Culquicondor <acondor@google.com>

content/en/examples/application/job/indexed-job-vol.yaml

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+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: 'indexed-job'
+spec:
+  completions: 5
+  parallelism: 3
+  completionMode: Indexed
+  template:
+    spec:
+      restartPolicy: Never
+      containers:
+      - name: 'worker'
+        image: 'docker.io/library/busybox'
+        command:
+        - "rev"
+        - "/input/data.txt"
+        volumeMounts:
+        - mountPath: /input
+          name: input
+      volumes:
+      - name: input
+        downwardAPI:
+          items:
+          - path: "data.txt"
+            fieldRef:
+              fieldPath: metadata.annotations['batch.kubernetes.io/job-completion-index']

content/en/examples/application/job/indexed-job.yaml

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+apiVersion: batch/v1
+kind: Job
+metadata:
+  name: 'indexed-job'
+spec:
+  completions: 5
+  parallelism: 3
+  completionMode: Indexed
+  template:
+    spec:
+      restartPolicy: Never
+      initContainers:
+      - name: 'input'
+        image: 'docker.io/library/bash'
+        command:
+        - "bash"
+        - "-c"
+        - |
+          items=(foo bar baz qux xyz)
+          echo ${items[$JOB_COMPLETION_INDEX]} > /input/data.txt
+        volumeMounts:
+        - mountPath: /input
+          name: input
+      containers:
+      - name: 'worker'
+        image: 'docker.io/library/busybox'
+        command:
+        - "rev"
+        - "/input/data.txt"
+        volumeMounts:
+        - mountPath: /input
+          name: input
+      volumes:
+      - name: input
+        emptyDir: {}