Document cross-namespace clusterRef for shadow links in Kubernetes

[david-yu]

Description

The Redpanda Operator supports an optional namespace field on clusterRef (redpanda-data/redpanda-operator#1562, backported to 26.1.x in redpanda-data/redpanda-operator#1663), which allows a ShadowLink to reference a source cluster in a different namespace on the same Kubernetes cluster.

This updates the Configure Shadowing in Kubernetes page (26.2) to:

• Add an example of a shadow link across namespaces on the same Kubernetes cluster using clusterRef.namespace.
• Warn that this same-cluster, cross-namespace topology is for development and testing only — a single Kubernetes cluster is a single failure domain, so production disaster recovery should place source and shadow clusters in separate Kubernetes clusters, ideally in separate cloud regions.
• Fix the note that previously said cross-namespace clusters require staticConfiguration, which is no longer the case.

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• Configure Shadowing in Kubernetes

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[david-yu]

@JakeSCahill could your review, this is for the beta.

[JakeSCahill]

Reviewed and fact-checked against the operator source and a live run. Two things:

This duplicates #1797 (opened yesterday for DOC-2274 without spotting this PR — sorry for the overlap). Closing #1797 in favor of this one since it came first and also covers the 26.1 backport in #1794. Porting over the one thing it had that this PR doesn't:

Suggestion — state the minimum operator version. On the 26.2 line the namespace field only exists from operator 26.2.1-beta.3 (the changelog entry ships there; beta.1/beta.2 don't have it, and an unknown CRD field is silently pruned, so on older betas the ShadowLink would quietly resolve the source in the wrong namespace). Suggest: "Requires Redpanda Operator 26.2.1-beta.3 or later" — or use the {operator-beta-version} attribute, which the build resolves to the latest beta (fallbacks for it are added in #1681/#1787).

Validation: I ran exactly this manifest shape end-to-end on kind on Monday (operator 26.2.1-beta.3, two single-broker clusters in source/shadow namespaces, enable_shadow_linking: true): the link reached Synced=True, the topic auto-created on the shadow cluster, and a produced message replicated with preserved offset and timestamp. Details in #1797. Everything else here matches the source: default resolution in the ShadowLink's namespace, field on sourceCluster.clusterRef, and the dev/test-only caveat — and the production line about connecting separate clusters with staticConfiguration is a good addition.