Note: Doc team updates the current version of the documentation and the
two previous versions (n-2), but we address *only high-priority, or
customer-reported issues* for -2 releases in support.
Describe the changes in the doc and link to your dev story:

1. - [x] Mandatory: Add the required version to the Fix version/s field.

2. - [x] Mandatory: Choose the type of documentation change or review.

• [ ] We need to update to an existing topic

• [ ] We need to add a new document to an existing section

• [x] We need a whole new section; this is a function not
  documented before and doesn't belong in any current section

• [ ] We need an Operator Advisory review and approval

• [ ] We need a z-Stream (Errata) Advisory and Release note for
  MCE and/or ACM

3. - [ ] Mandatory: Find the link to where the documentation update
should go and add it to the recommended changes. You can either use the
published doc or the staged repo for this step:

Note: As the feature and doc is understood, this recommendation may
change. If this is new documentation, link to the section where you think
it should be placed.

Customer Portal published version

https://docs.redhat.com/en/documentation/red_hat_advanced_cluster_management_for_kubernetes/2.13/html/clusters/cluster_mce_overview#mce-network-configuration 

Doc staged repo within the ACM Workspace:
https://github.com/stolostron/rhacm-docs/tree/2.13_stage/clusters/assisted_installer

4. - [x] Mandatory for GA content:

• [x] Add steps, the diff, known issue, and/or other important
  conceptual information in the following space:

• [x] * Required access level: *Cluster
  Administrator* 

• [x] Add verification at the end of the task, how does the user
  verify success (a command to run or a result to see?)

• [x] Add link to dev story here:

5. - [x] Mandatory for bugs: What is the diff? Clearly define what the
problem is, what the change is, and link to the current documentation. Only
use this for a documentation bug.

Description:
Lately we’ve introduced support for per-cluster mirror registry in the Assisted Installer. This feature allows each AgentClusterInstall and each }}{{InfraEnv to reference a dedicated mirror registry configuration object, enabling users to configure and manage custom registries on a per-cluster basis.

Scope of Documentation:

• Overview of the per-cluster mirror registry concept and its benefits.

• How to define and apply a MirrorRegistryConfiguration resource.

• How to link it to an AgentClusterInstall and  InfraEnv via the new reference field.

• YAML examples for both the mirror config and the AgentClusterInstall.

• Behavior when the configuration is invalid or missing.

• Notes about backward compatibility (e.g., global mirror still applies if per-cluster is not set).

Relevant links:

• Pull request - link
• Documented user-guide in assisted-installer repository - link
• Assisted-Installer epic - link