Document promoting a bundle use case

Describe the problem/challenge you have
I wanted to know how to use imgpkg when promoting a bundle between various environments (dev->qa->prod). And have a deeper understanding on how `imgpkg pull` with `imgpkg copy` helps with this.

Describe the solution you'd like
A new scenario added to help demonstrate this

Note: Would be useful to have diagrams to help communicate this. such as https://jvns.ca/blog/2017/06/04/learning-about-kubernetes/

cc/ @iamhsk

Review the existing imgpkg scenario documentation and the `imgpkg pull` and `imgpkg copy` entry points first. Add a scenario showing bundle promotion through dev, QA, and prod, with diagrams explaining the flow and a clear demonstration of the commands.

Written by the indexing model from the issue text.