Document how to setup authorization

[jrarmstro]

Is your feature request related to a problem? Please describe.
There should be more documentation explaining how to setup authorization in the OPAL client. There's plenty of documentation already available for authentication, but very little to do with authorization.

Describe the solution you'd like
The official OPAL docs webpages should contain a section devoted to Rego authorization.

Describe alternatives you've considered
There already exists a Docker example of setting up authorization: https://github.com/permitio/opal/blob/master/docker/docker-compose-with-oauth-initial.yml

Additional context
The Helm chart should include docs about authorization as well; permitio/opal-helm-chart#56