k3s - update on the air-gap installation instruction

[btat]

Original issue: https://github.com/rancher/docs/issues/2351

Copying over relevant bits from original issue since K3s docs have been moved off the old rancher/docs repo.

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During the validation for this issue: rancher/rancher#25495 (comment)

I found there are some improvements we can do for the documentation for k3s air-gap installation: https://rancher.com/docs/k3s/latest/en/installation/airgap/

• for the section Prepare Images Directory, add some explanation for why we still need this when we have a private registry
• for the section Create Registry YAML, add some explanation for when and how the private registry will be used by k3s, why the private registry is not used for k3s' own images
• for the section Install K3s, add a sample command when installing k3s with an external database

[rancher-max]

Adding to this, we may also want to document that it is okay and sometimes advisable to remove the *.tar* files after initial startup when doing the tarball airgap method. This is because, when those files are present, restarts of the systemd service can take a long time to unpack those files since startup doesn't know that what is inside those files hasn't changed since the last start.