Document image mirroring via Extra-Manifests for the SiteConfig Operator.

Always look at the current documentation to describe the change that is needed. Use the source or portal link for Step 4:

• Use the Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes
• Use the GitHub link to find the staged docs in the repository: https://github.com/stolostron/rhacm-docs

Describe the changes in the doc and link to your dev story
Provide info for the following steps:
1. - [x] Mandatory Add the required version to the Fix version/s field.
2. - [x] Mandatory Choose the type of documentation change.

• [ ] New topic in an existing section or new section
• [x] Update to an existing topic
  3. - [ ] Mandatory for GA content:
• [x ] Add steps and/or other important conceptual information here:
  https://docs.redhat.com/en/documentation/red_hat_advanced_cluster_management_for_kubernetes/2.12/html/multicluster_engine_operator_with_red_hat_advanced_cluster_management/siteconfig-intro#install-create-extra-manifests

This section of the document should call out that users performing installations in disconnected environments should supply their mirroring configurations (ImageDigestMirrorSet manifests) as an extra manifest