How to configure private registry for curated packages

**What would you like to be added**:

EKS Anywhere documentation should add instructions on how to configure private registry for curated packages.

**Why is this needed**:

EKSA support air gapped clusters. As a EKSA License Subscription customer, I want to install curated packages from private registry.

Start by locating the EKS Anywhere documentation covering curated packages and air-gapped clusters, then review the existing registry configuration guidance. Add instructions for configuring a private registry so subscribed customers can install curated packages in an air-gapped cluster; the documentation should clearly describe the required configuration and usage.

Written by the indexing model from the issue text.