[admin-api] Write documentation for rbac

[aaperis]

Please describe the feature

As a systems administrator, I want documentation on how to configure the RBAC file so that the helpdesk and repository admins can do their job.

Acceptance criteria

• Documentation on how the rbac.json can look like with privilege separation for admins, helpdesk and one extra example for a user with read only access.

Additional context

Estimation of size

small/medium/big

Estimation of priority

low/medium/high

[jbygdell]

Changed to admin-api since that's where this should be.