Create documentation on postrender to configure package

Mimir employs a wrapper patter that is different from most Big Bang packages that are forked. To make changes, the postrender approach is used. Some documentations should be generated for maintainers to easily make changes as part of knowledge transfer and onboarding.

Acceptance Criteria

• Background on the wrapper chart pattern
• How to implement post render w/ example of how it is being implemented for Miimr
• Limitation of post render (refer to helm doc)
• Link to additional documentation on helm/kustomize/flux on post rendering approach for more info