Document stackset role pair concept

[abukharov]

This configuration is used on a stackset to define the IAM role pair used for provisioning:

    admin_role_arn: arn:aws:iam::822924222578:role/service-role/AWSControlTowerStackSetRole
    exec_role_name: AWSControlTowerExecution

Currently it's missing from the documentation, can we please document it. Here's the code that handles it: https://github.com/Innablr/cloudformation-seed/blob/develop/cloudformation_seed/util.py#L415

[abukharov]

we can have it documented that either;
a. The manually expected admin and execution roles exist
b. You have a flag/param available in CFN guard to use the Org StackSet role
c. Doco custom deploy/exec roles