Request for documentation

One of the ClusterInstance fields under .spec is extraLabels. This appears to be undocumented. Existing AI and IBI templates do not make use of this field either, but for our Hosted Control Planes testing, we are applying labels to our hosted clusters using this feature.

Prerequisite: Start with what we have

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#default-templates

search yields no matches for `extraLabels`

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.

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

3. - [x] Mandatory for GA content:
            
       - [x] Add steps and/or other important conceptual information here: 
       
The clusterInstance.spec.extraLabels field is not (as one expects at first look) simply a dictionary of labels, but a dictionary of arbitrary strings that map to dictionaries of labels. Some explanation of this breakdown would be appreciated.

e.g. I created:
extraLabels:
ManagedCluster:
common: "true"
label-a : "value-a"
            
       - [ ] Add Required access level for the user to complete the task here:
       
same access as creator of clusterInstance

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

It likely will be required to add code to the AI and IBI cluster-wide templates to ensure labels are propagated. Current templates do not use `Spec.ExtraLabels` at all.    
     
       - [ ] Add link to dev story here:

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