[doc] Image Registry to support ODF external mode as backend storage

1. Proposed title of this feature request

document how to use ODF external mode to configure image registry.

2. What is the nature and description of the request?

currently our documentation https://docs.openshift.com/container-platform/4.12/registry/configuring_registry_storage/configuring-registry-storage-baremetal.html#registry-configuring-registry-storage-rhodf-cephfs_configuring-registry-storage-baremetal only mentions how to leverage ODF internal mode for image registry backend storage. It does not mention how to use ODF external mode to do the same.

3. Why does the customer need this? (List the business requirements here)

Currently a lot of customer (especially in China market) all the ODF users are using ODF external mode. They don't have ODF internal mode. But there are lack of documents to guide them how to configure image registry. 

4. List any affected packages or components.
only image registry part of documentation are affected. According to my local sanity test, there is no development effort.