Finalize "extra package" documentation

This is largely a cleanup / validation issue for the existing doc regarding "extra package" deployment: https://repo1.dso.mil/big-bang/bigbang/-/blob/master/docs/guides/deployment-scenarios/extra-package-deployment.md

Steps we should take:

1. Decouple the documentation for package/wrapper. We should separate the docs out so that there is a focus on extra package deployment, without any wrapper interactions, and a second piece focused on the wrapper functionality. This could be in the same document, or link to docs inside of the wrapper repo.
2. Clarify OCI vs Git? Both are supported but OCI should be encouraged.
3. Remove the warning at the top of the doc.