docs: specify minimum number of nodes in k8s cluster

[a-h]

Minor update. The Helm chart can't be deployed to a single Node k8s setup due to pod affinity rules in the chart.

[CLAassistant]

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✅ JStickler
❌ a-h
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[JStickler]

@a-h Thanks for the update. Could you please sign the Contributor License Agreement, then we can get this merged.

[grafanabot]

This PR must be merged before a backport PR will be created.