Move `remove` instructions into Quick Start

[bmicklea]

Describe the bug It's relatively easy to fat finger the tag when creating your ModelKit, but when you're going through the Quick start you don't know how to use remove. Remove should be explained to when you're doing your first pack / list instead of in the Next Steps which people may not read. Also OCI registry isn't a commonly understood term outside DevOps - would be good to demystify it.

To Reproduce

• Move examples to GitHub for registry
• Explain that OCI registry == conatiner registry or GitHub registry packages

[bmicklea]

Brett is going to cover this with his signing changes (#313 ):

• Add cosign to Next Steps
• Move remove instructions to Quick Start immediately following the first push (in case people make a mistake)
• Update QS references from Docker.io to GHCR for consistency