W-17917573: Document nodeSelector and affinity Helm properties for RTF-kt

[kevintroller]

Add global.deployment.nodeSelector and global.core.affinity to the Optional Parameters table and values.yml example in install-helm.adoc. These properties allow customers to control pod scheduling for RTF core services using Kubernetes node selectors and affinity rules.

Note: Pending dev team confirmation on W-18022835 before merge.

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