docs: add warning about multi-namespace Spark Operator deployments

[Sanika123-art]

What this PR does

Adds an Important alert to the Spark Operator documentation explaining that
the operator creates cluster-scoped resources (such as webhooks and ClusterRoles),
which makes running instances in different namespaces within the same cluster
unsupported.

Why this change is needed

Users attempting to deploy multiple Spark Operator instances may encounter
resource conflicts that are not clearly documented today. This change makes the
limitation explicit and guides users toward the recommended approach of using a
single operator namespace with different Spark job namespaces.

Related issue

• Feedback: docs/components/spark-operator/user-guide/running-multiple-instances-of-the-operator.md ([Feedback] docs/components/spark-operator/user-guide/running-multiple-instances-of-the-operator.md #4256)

[google-oss-prow]

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[google-oss-prow]

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[Arhell]

/ok-to-test