Update doc to use the new import images command

[jiayiwang7]

What would you like to be added:

We deprecated a couple of CLI commands: https://github.com/aws/eks-anywhere/blob/main/designs/air-gapped.md#deprecated-commands in favor of the new import images command. We need to reflect it in our official doc: https://anywhere.eks.amazonaws.com/docs/reference/clusterspec/optional/registrymirror/#import-images-into-a-private-registry

Why is this needed: