add docs on migrating to new ASO API

[nojnhuh]

What type of PR is this?
/kind documentation

What this PR does / why we need it:
This PR adds docs describing how to migrate from AzureManagedControlPlane to AzureASOManagedControlPlane.

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #5032

Special notes for your reviewer:

• cherry-pick candidate

TODOs:

• squashed commits
• includes documentation
• adds unit tests

Release note:

Added docs describing how to migrate existing AKS clusters managed by CAPZ to the new ASO-based API

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by:
Once this PR has been reviewed and has the lgtm label, please ask for approval from nojnhuh. For more information see the Kubernetes Code Review Process.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these files:

• OWNERS

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[nojnhuh]

This bug has been fixed: Azure/azure-service-operator#3805

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.53%. Comparing base (78e1385) to head (d711276).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5305   +/-   ##
=======================================
  Coverage   52.53%   52.53%           
=======================================
  Files         272      272           
  Lines       29372    29372           
=======================================
  Hits        15432    15432           
  Misses      13143    13143           
  Partials      797      797           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests