Document the set of least-required permissions required by Toolkit

**Is your feature request related to a problem? Please describe.**
* it should be easy for users to understand what permissions are required for all features provided by the Toolkit (for example: allow `lambda:ListFunctions` is required so the toolkit can show all Lambda Functions)
* it should be easy (automated or guided walk through) for users to set up these permissions

**Describe the solution you'd like**
Possible ideas, open for discussion:
* An AWS Managed policy
* living documentation, in a markdown file within the repo, containing JSON representing the policy, and instructions on how to define the policy, create a user, and associate user with the policy (instructions likely web console based, but maybe aws cli too)
* as new/inbound code uses new service calls, the json policy must be updated as well

Start by reviewing the Toolkit’s feature set and the AWS service calls each feature requires. Define the least-privilege JSON policy in a repository markdown file, with web-console and/or AWS CLI instructions for creating a user and associating the policy. Done means the permissions cover all Toolkit features and the setup path is clear.

Written by the indexing model from the issue text.