Simplify the document cloud example

[shaobo-he-aws]

Issue #, if available:

Description of changes:
This PR simplifies the original model as follows.

1. Remove is_authenticated attribute from context. Now we assume each user is authenticated before requesting authorization.
2. Remove Public entity type. I think it should amount to unconstrained principal.
3. Remove DocumentShare entity type. It appears to me that Group is all we need, provided that we remove the Public type.
4. Remove isPrivate attribute. My rationale is that it can conflict with the public_access attribute.
5. Merge certain policies into a single policy.
6. Remove two forbid policies using stale attributes.
7. Revise the remaining policy such that only resource owner's blocked list takes effects.

[shaobo-he-aws]

@khieta @aaronjeline I'll update the entity JSON file and request JSON files if the policies/schema look good to you.

[khieta]

Left some small comments, but overall the edits seem fine to me. Will delay approval until the README and entity/request JSON files are updated.

[khieta]

Looks good. Just left some small comments.

[shaobo-he-aws]

Closed for now unless there are new interests.