Update airgapped docs to allow for completely airgapped admin machine

**What happened**: EKS-A docs [airgapped installation page](https://anywhere.eks.amazonaws.com/docs/getting-started/airgapped/) indicates admin machine must be connected to internet to download artifacts (`eksctl`CLI, images, and dependencies).

**What you expected to happen**: EKS-A required artifacts can be downloaded and packed into tarballs on an internet-connected (non-admin) machine, and then transferred by USB or other method to a completely airgapped admin machine. The docs should indicate this is possible and provide some pointers on how to accomplish it.

**How to reproduce it (as minimally and precisely as possible)**: Demonstration of this use case available on internal Amazon network.

**Anything else we need to know?**:

**Environment**:
- EKS Anywhere Release: latest (docs only)
- EKS Distro Release: latest (docs only)

Start with the EKS-A airgapped installation page linked in the issue and verify which artifacts are currently described as requiring internet access on the admin machine. Document the supported workflow for downloading and transferring artifacts from a separate connected machine, including pointers to the relevant tarballs and dependencies; done when the page clearly covers a completely airgapped admin machine.

Written by the indexing model from the issue text.