Add documentation for Groups, Users and RBAC

Currently, we don't have any information about groups and RBAC:
[OpenBAS User Documentation](https://docs.openbas.io/latest/administration/users/?h=user)

A first PR for users was submitted earlier, but it did not include information about users, groups, or RBAC:
[PR #63](https://github.com/OpenBAS-Platform/docs/pull/63)