Create documentation of Keycloak Admin interface

[JanssenBrm]

We should create a guide for a project administrator on how they can manage users in the Keycloak admin interface. This guide should show how these administrators can add and remove users to specific roles. Additionally, we should also showcase how each role affects the user's permission on each of the services.

This is linked to the work done in https://github.com/ESA-APEx/Terraform-Keycloak/issues/6