Document new location of docker images

[carolynvs]

What does this change

A while back we switched to only publishing images to ghcr.io instead of docker.io due to changes in the Hub service (rate limits, etc). I forgot to update these pages that document the images when I did that, so this fixes the doc to point to the current image locations.

What issue does it fix

Closes #2262

Notes for the reviewer

I've opened a follow-up issue to consider if it's worth supporting these images (porter, and workshop) in v1.
#2265

Checklist

• Did you write tests?
• Did you write documentation?
• Did you change porter.yaml or a storage document record? Update the corresponding schema file.
• If this is your first pull request, please add your name to the bottom of our Contributors list. Thank you for making Porter better! 🙇‍♀️

Reviewer Checklist

• Comment with /azp run test-porter-release if a magefile or build script was modified
• Comment with /azp run porter-integration if it's a non-trivial PR