Add example showing reuse of k8s secret for executors (#731)

### Checklist

- [ ] Follow the [manual testing
process](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/TEST.md)
- [ ] Update
[changelog](https://github.com/sourcegraph/deploy-sourcegraph-helm/blob/main/charts/sourcegraph/CHANGELOG.md)
- [ ] Update [Kubernetes update
doc](https://docs.sourcegraph.com/admin/updates/kubernetes)

### Test plan

Adding examples to doc files

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Author: marcleblanc2

charts/sourcegraph-executor/k8s/examples/private-image-registry/override.yaml

@@ -0,0 +1,15 @@
+## Override file demonstrating the use of a private docker image registry and custom image names
+
+sourcegraph:
+  image:
+    repository: custom-repo.ecr.us-east-1.amazonaws.com/sourcegraph
+
+  # Registry credentials: https://kubernetes.io/docs/tasks/configure-pod-container/pull-image-private-registry
+  imagePullSecrets:
+  - name: regcred # Pre-existing secret that contains the credentials needed to pull from the image registry
+
+# Allow Executors to reuse the same registry credentials to pull the needed images for Batch Changes and Auto-indexing
+executor:
+  extraEnv:
+   - name: KUBERNETES_IMAGE_PULL_SECRETS
+     value: regcred

charts/sourcegraph/examples/private-image-registry/override.yaml

@@ -25,3 +25,9 @@ alpine:
   image:
     name: alpine # Use a custom name instead of the Sourcegraph name - i.e. alpine-3.12
     tag: 3.12 # Use a custom tag instead of the default - i.e. 3.35.1
+
+# Allow Executors to reuse the same registry credentials to pull the needed images for Batch Changes and Auto-indexing
+executor:
+  extraEnv:
+   - name: KUBERNETES_IMAGE_PULL_SECRETS
+     value: regcred