[Cedar Doc Issue] Action Groups Support

[apandays]

We need a holistic pass on action groups with respect to the Cedar user guide, and add it to atleast the below

1. Defining Schema
2. Policies Section
3. Best Practices section
   3.1 How to use action groups for Role based Access

[bisdavid]

Thanks for reporting this, Abhishek! On my todo list. :-)