[Docs] Missing authentication annotation for packageinstall CR

[rguske]

Update the section which describes the offline installation of Tanzu Packages with the missing information how to configure a registry secret for the packageinstall CR.

Example:

---
apiVersion: packaging.carvel.dev/v1alpha1
kind: PackageInstall
metadata:
  name: simple-app-with-secret
  annotations:
    ext.packaging.carvel.dev/fetch-0-secret-name: simple-app-secret

Source: Authenticating to Private Registries