docs: clarify gp3 StorageClass tuning for Helm installs

[0lut]

Summary

• document EKS gp3 usage through persistence.storageClass, keeping StorageClass ownership outside the app chart
• explain that gp3 IOPS and throughput come from the selected StorageClass, including AWS baseline values and a tuned example
• add Helm NOTES guidance for clusters without a default StorageClass

Validation

• helm lint deploy/helm/gitmirrorcache --set config.objectStore.s3.bucket=ci-bucket
• helm install git-cache deploy/helm/gitmirrorcache --dry-run=client --set config.objectStore.s3.bucket=ci-bucket
• helm template git-cache deploy/helm/gitmirrorcache --set config.objectStore.s3.bucket=ci-bucket --set persistence.storageClass=gp3

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✅ Devin Review: No Issues Found

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