docs: mention DWO_NAMESPACE env var in backup docs (#3059)

Signed-off-by: dkwon17 <dakwon@redhat.com>

Author: dkwon17

modules/administration-guide/pages/devworkspace-backup-integrated-openshift-registry.adoc

@@ -45,3 +45,5 @@ echo "https://$(oc get route default-route -n openshift-image-registry --templat
 
 Once the backup job is finished, the backup archives will be available in the {devworkspace} {namespace} under a repository
 with a matching {devworkspace} name.
+
+include::partial$snip_defining-dwo-namespace-for-backups.adoc[]

modules/administration-guide/pages/devworkspace-backup-regular-oci-registry.adoc

@@ -54,3 +54,5 @@ kubectl label secret devworkspace-backup-registry-auth controller.devfile.io/wat
 ====
 The {devworkspace} Operator copies the `devworkspace-backup-registry-auth` secret to each {devworkspace} {namespace} so that backups from user workspaces can be pushed to the registry. If you do not want that secret copied automatically, create a `devworkspace-backup-registry-auth` secret with user-specific credentials in each {devworkspace} {namespace} instead.
 ====
+
+include::partial$snip_defining-dwo-namespace-for-backups.adoc[]

modules/administration-guide/partials/snip_defining-dwo-namespace-for-backups.adoc

@@ -0,0 +1,22 @@
+:_content-type: SNIPPET
+
+[NOTE]
+====
+If the installation {namespace} for the {devworkspace} operator is not `openshift-operators`, you must define it as an environment variable for the {prod} dashboard in the `CheCluster` Custom Resource.
+
+[source,yaml,subs="+attributes,+quotes"]
+----
+apiVersion: org.eclipse.che/v2
+kind: CheCluster
+spec:
+  components:
+    dashboard:
+      deployment:
+        containers:
+          - env:
+              - name: DWO_NAMESPACE
+                value: $OPERATOR_INSTALL_NAMESPACE <1>
+            name: che-dashboard
+----
+<1> For Red Hat OpenShift, the default installation namespace for the {devworkspace} operator is `openshift-operators`. See the xref:devworkspace-operator.adoc[{devworkspace} operator overview].
+====