Add documentation for new experimental command `pack manifest`

[jjbustamante]

Summary

Pack 0.34.0 is expected to be released very soon, in this new version we are implementing the RFC 0124, it involves the creation of a new set of commands pack manifest to handle Image Indexes in a registry.

What questions should this documentation answer?

• For each new command, we should answer:
  • How do we use the feature?
  • In which use cases the new command can help me?

Who is the target audience for this documentation?

• App Developer
• Buildpack Author
• Operator
• Other:

Additional Information

[ChiragAgg5k]

I would like to work on writing this documentation. Can you assign it to me?