Add Documentation for RBD Async Disaster Recover

[Madhu-1]

Is this a bug report or feature request?

• Feature Request

What should the feature do:
Rook already support Enable Mirroring between two ceph clusters. The user can choose to either enable Pool Level mirroring or Image level mirroring. In case if the user wants to Use Image level mirroring he need to perform few steps. Rook needs to document the steps required for the same.

What is use case behind this feature:

This is very useful in case of Disaster recovery the important data in the PV will be mirrored to the remote site. If the primary cluster goes down the admin should be able to move the kubernetes workload from the primary cluster to the remote cluster.
Environment:

[Madhu-1]

/assign @Yuggupta27

[github-actions]

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