BB Extension: Document way to consume/translate BB values

Document the nested HelmRelease solution that was proposed in #1457 (closed) which provides a way for packages to consume/translate BB values to package defaults.

This documentation should live in or alongside the existing guides for package deployment (https://repo1.dso.mil/big-bang/bigbang/-/blob/master/docs/guides/deployment-scenarios/extra-package-deployment.md).