Document Kubernetes RBAC permissions

[dhkim1920]

Document Kubernetes RBAC permissions for the CNCF Kubernetes provider.

This adds a new guide describing the Kubernetes API permissions needed by KubernetesExecutor, KubernetesPodOperator, KubernetesJobOperator, cleanup jobs, and custom resource operators. It also links the guide from related Kubernetes provider docs.

closes: #40210

Validation performed:

• prek run --files providers/cncf/kubernetes/docs/kubernetes_rbac.rst providers/cncf/kubernetes/docs/index.rst providers/cncf/kubernetes/docs/kubernetes_executor.rst providers/cncf/kubernetes/docs/local_kubernetes_executor.rst providers/cncf/kubernetes/docs/operators.rst
• git diff --check
• breeze build-docs --docs-only cncf.kubernetes

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Was generative AI tooling used to co-author this PR?

• Yes — Codex (GPT-5)

Generated-by: Codex (GPT-5) following the guidelines

[boring-cyborg]

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