Improve the documentation around Security Model

[vibhas]

Describe the problem/challenge you have We need to improve our documentation around Security Model to clarify the following things for our users:

• The current Security Model doc is focused on AppCR. We should add more details with respect to the new Packaging APIs as well.
• We should add more clarification that the Package consumers need to create and provide the Service accounts while installing the package. Probably we can more details to this section of the docs.
• Currently, package consumers do not have a good way to learn about the permissions that are required by a package. We are planning to improve this experience post MVP (#142). In the same section of the docs as above, we should provide details that the experience around discovering package permissions will be improved in the future.