Update documentation for Packer to include minimum operational requirements

[tpdownes]

Building images with Packer is a significant challenge because there are minimum requirements that are not always fully captured in a blueprint. These include:

1. the IAM roles of the user invoking Packer
2. the IAM roles of the service account used by the build VM
3. inbound internet access to the build VM
4. outbound internet access from the build VM

We document these requirements and capture several of them in an updated copy of the Packer image-builder.yaml example.

I suggest reviewing this commit-by-commit as the last commit includes a number of formatting changes made automatically by mdformat that do not include functionality changes.

Submission Checklist

Please take the following actions before submitting this pull request.

• Fork your PR branch from the Toolkit "develop" branch (not main)
• Test all changes with pre-commit in a local branch #
• Confirm that "make tests" passes all tests
• Add or modify unit tests to cover code changes
• Ensure that unit test coverage remains above 80%
• Update all applicable documentation
• Follow Cloud HPC Toolkit Contribution guidelines #