Add airgap example

[carolynvs]

What does this change

Add an example of shoving a bundle over an airgap. 🐳 💨🚜

TODO:

• Selectively replace the org when moving images #1047 (comment)

What issue does it fix

Needed it for a demo and people asked a lot about it in Slack.

Notes for the reviewer

• As I wrote this I realized that our docs don't explain how to do this, so there will be follow-up PRs.
• Found a bug in how we rename images when we copy them to another registry, need to split that out into another PR. It's in here because it broke my demo...

Checklist

• Unit Tests
• Documentation
• Schema (porter.yaml)

[carolynvs]

@vdice Okay this is finally ready for review

[vdice]

🐳