Documentation on how to configure CAPZ clusters to pull from a Private ACR registry

[jsturtevant]

/kind doc

[Before submitting an issue, have you checked the Troubleshooting Guide?]

What steps did you take and what happened:
[A clear and concise description of what the bug is.]
Add documentation on how to configure kubelet in CAPZ to pull from a Private ACR registry. It is alluded to here https://capz.sigs.k8s.io/topics/vm-identity.html?highlight=acrpull#how-to-use-managed-identity but there aren't detailed docs on the exact set up

What did you expect to happen:
Make it easy to configure CAPZ clusters to use private ACR registry for management and workload clusters.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• cluster-api-provider-azure version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[k8s-ci-robot]

@jsturtevant: The label(s) kind/doc cannot be applied, because the repository doesn't have them.

In response to this:

/kind doc

[Before submitting an issue, have you checked the Troubleshooting Guide?]

What steps did you take and what happened:
[A clear and concise description of what the bug is.]
Add documentation on how to configure kubelet in CAPZ to pull from a Private ACR registry. It is alluded to here https://capz.sigs.k8s.io/topics/vm-identity.html?highlight=acrpull#how-to-use-managed-identity but there aren't detailed docs on the exact set up

What did you expect to happen:
Make it easy to configure CAPZ clusters to use private ACR registry for management and workload clusters.

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• cluster-api-provider-azure version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

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