Add more useful apply examples to docs/intro/concepts/inputs-outputs/#apply

[stack72]

The examples that are there right now are basic usage of apply - it would be good to do something where we can do something like build an IAM Policy in our apply to show a more complex example

This relates back to https://github.com/pulumi/pulumi/issues/5236 and the incorrect use of the apply inside the get_policy_document invoke