W-21093225: Document immutable Helm properties after initial RTF

[kevintroller]

Certain Helm values (rtfRegistry, pullSecretName, provider, repositoryNamespace) are written to the generated-properties secret only during the first agent install. Changes made post-activation are not reflected, which can cause image pull errors or incorrect provider configurations. Added IMPORTANT admonitions to install-helm.adoc and configure-local-registry.adoc so users set these values correctly before first activation.

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