Create an informative issue (See each section, incomplete templates/issues won't be triaged)

Using the current documentation as a model, please complete the issue template. 

Note: Doc team updates the current version and the two previous versions (n-2). For earlier versions, we will address only high-priority, customer-reported issues for releases in support.

Prerequisite: Start with what we have

Always look at the current documentation to describe the change that is needed. Use the source or portal link for Step 3:

 - Use the Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes/2.6
https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes/2.5/html-single/clusters/index#enable-cim

Section: 1.5.2. Enabling the Central Infrastructure Management service

 - 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. - [ ] Mandatory Choose the documentation release (Published releases are refreshed ~weekly after publication). 
     
      - [ ] ACM 2.4
      - [X] ACM 2.5
      - [X] ACM 2.6
      - [X]ACM 2.7 (upcoming release)

      - [ ] MCE 2.0
      - [ ] MCE 2.1 
      - [ ] MCE 2.2 (upcoming release)
   

2. - [ ] Mandatory Choose the type of documentation change.

      - [ ] New topic in an existing section or new section
      - [X] Update to an existing topic

3. - [X] Mandatory for bugs: What is the diff? Clearly define what the problem is, what the change is, and link to the current documentation:

Add steps to declare the mirrorRegistry while enabling cim with ACM for disconnected environments

This is not required for connected environments, so we could either create a separate doc for enabling cim for disconnected mode or accommodate this step into existing doc indicating that this additional step is required if the environment is disconnected/airgapped.

Doc Section:
1.5.2. Enabling the Central Infrastructure Management service

Doc link:
https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes/2.5/html-single/clusters/index#enable-cim

Doc link to the missing parameter:
https://docs.openshift.com/container-platform/4.10/scalability_and_performance/ztp-deploying-disconnected.html#ztp-configuring-the-cluster-for-a-disconnected-environment_ztp-deploying-disconnected

If you check the "AgentServiceConfig" example in above doc, you will see that it imports a configmap with "mirrorRegistryRef". This is required if the environment is disconnected.

4. - [ ] Mandatory for GA content:
            
       - [ ] Add steps and/or other important conceptual information here: 
       
            
       - [ ] Add Required access level for the user to complete the task here:
       

       - [ ] Add verification at the end of the task, how does the user verify success (a command to run or a result to see?)
     
     
       - [ ] Add link to dev story here:

   - Errata release note version, this is NOT for regular refreshes: 
      (published with Errata release)
  
      - [ ] ACM 2.4.x
      - [ ] ACM 2.5.x
      - [ ] ACM 2.6.x

      - [ ] MCE 2.0.x

      - [ ] MCE 2.1.x

5. Optional: Get back to issue templates to create more issues for the doc team: https://github.com/open-cluster-management/backlog/issues/new/choose