Enhance create cluster from backup to highlight dependency on customer CA cert from original cluster

In this section of the doc:

https://github.com/aws-samples/aws-cloudhsm-cloudformation-template#creating-a-cloudhsm-cluster-from-a-backup

Make clear that the customer CA cert associated with the cluster from which the backup was taken must exist in Secrets Manager under the name:

```
/{system_id}/{backup_cluster_id}/customer-ca-cert
```

Where `{system_id}` is the value of the `pSystem` parameter used for both the stack associated with the original cluster from which the backup was taken and the new cluster to be created from the backup. Also highlight that the `pSystem` parameter value for both stack needs to be the same.

The original customer CA cert is used during the process of creating a new cluster from a backup to configure the EC2 client with the proper CA cert so that the CloudHSM client tools can interact with HSMs in the newly created cluster.