Add access control docs.

[uraj]

Description

Some simple documentations explaining how Teaclave access control model works.

Fixes # (issue)

Type of change (select or add applied and delete the others)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• API change with a documentation update
• Additional test coverage
• Code cleanup or just sync with upstream third-party crates

How has this been tested?

Checklist

• Fork the repo and create your branch from master.
• If you've added code that should be tested, add tests.
• If you've changed APIs, update the documentation.
• Ensure the tests pass (see CI results).
• Make sure your code lints/format.

[mssun]

Merged. Thank you. This is very helpful!