docs: mention DWO_NAMESPACE env var in backup docs

[dkwon17]

What does this pull request change?

Adds a notice about the DWO_NAMESPACE env var which is needed for the dashboard to detect backup status. This is needed only when the DWO install namespace is not openshift-operators.

What issues does this pull request fix or reference?

Specify the version of the product this pull request applies to

DS 3.27.

Pull Request checklist

The author and the reviewers validate the content of this pull request with the following checklist, in addition to the automated tests.

• Any procedure:
  • Successfully tested.
• Any page or link rename:
  • The page contains a redirection for the previous URL.
  • Propagate the URL change in:
    • Dashboard default branding data
• Builds on Eclipse Che hosted by Red Hat.
• the Validate language on files added or modified step reports no vale warnings.

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[dkwon17]

This value is the same for both upstream and downstream.

[gtrivedi88]

Thanks for adding the DWO_NAMESPACE documentation — this is useful context for non-default operator installations.

A few CQA/style issues to address:

1. Bold pseudo-heading — **Note:** must be a proper [NOTE] admonition block (DITA compatibility, Vale BlockTitle rule)
2. Bare placeholder — $OPERATOR_INSTALL_NAMESPACE in the snippet has no explanation; the reader needs guidance on what value to substitute
3. Snippet placement — In the regular OCI registry file, the include breaks the flow between the callout and authSecret explanation; consider moving it to the end
4. Missing final newline — The snippet file should end with a newline