Related to slack discussion https://redhat-internal.slack.com/archives/C0282HW2YHZ/p1673471803026709

The setup configuration for a disconnected env is slightly different than the regular installation. We need to point to differences in the doc

Update this section
https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes/2.6/html/install/installing#install-on-disconnected-networks

(another option is to add the info to the Prereqs section
https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes/2.6/html/backup_and_restore/backup-intro#prerequisites-backup-restore )

And add the following subsection ( at the same level with 1.4.8 ):
1.4.8 Additional considerations when using backup and restore component
Content:
Complete the following additional steps when you enable the backup and restore component with disconnected Red Hat OpenShift Container Platform:
Update the MultiClusterHub resource with this annotation to override the source from which the OADP operator will be installed. This annotation should be created before the `cluster-backup` component is enabled on the MultiClusterHub resource.

```
apiVersion: operator.open-cluster-management.io/v1
kind: MultiClusterHub
metadata:
  annotations:
    installer.open-cluster-management.io/oadp-subscription-spec: '{"source": "redhat-operator-index"}'
```

The `redhat-operator-index` is a custom name and represents the name of the CatalogSource resource defined by the user and used to access redhat operators in the disconnected environment.

oc get catalogsource -A
```
NAMESPACE               NAME                         DISPLAY                       TYPE   PUBLISHER   AGE
openshift-marketplace   acm-custom-registry          Advanced Cluster Management   grpc   Red Hat     42h
openshift-marketplace   multiclusterengine-catalog   MultiCluster Engine           grpc   Red Hat     42h
openshift-marketplace   redhat-operator-index                                      grpc               42h
```

Provide info for the following steps:

1. - [ ] *Mandatory* Choose the documentation release (Published releases are refreshed ~weekly after publication). 
     
      - [ ] ACM 2.4
      - [ ] ACM 2.5
      - [x] ACM 2.6 
      - [x] ACM 2.7 (upcoming release)
      - [ ] MCE 2.0
      - [ ] MCE 2.1
      - [ ] MCE 2.2 (upcoming release)
   

2. - [ ] *Mandatory* Choose the type of documentation change.

      - [ ] New topic in an existing section or new section
      - [x] Update to an existing topic