Docs: Section on Upgrading to Packaging CRs from Existing App CRs

Users of kapp-controller who have been using App CRs for existing workloads may eventually want to change to using packaging CRs with kapp-controller. There should be kapp-controller docs created that point out key factors to accomplish this update. The use case of these docs is captured in #184 and this issue may depend on the outcome of this [comment in #184](https://github.com/vmware-tanzu/carvel-kapp-controller/issues/184#issuecomment-831288162).

Some things to consider when writing these docs are:
* How to replicate the App spec when creating a Package CR in order to preserve the existing App CR?
* How InstalledPackages know what App to use if the App CR already exists (i.e. the InstalledPackage should have the same name as the App CR)
* Where to create the InstalledPackage (i.e. in the App CR namespace)