Add managed cluster documentation for downstream images

[SimonBaeumer]

Description of the change: Add documentation how to add a managed cluster by using downstream images.

Motivation for the change: It does not work to deploy managed clusters by following the documentation because some more tweaks are necessary.

A script may follow in the future but first I want to discuss the documentation and having an approve that this approach is correct.

[SimonBaeumer]

/assign @gurnben

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Kyl-Bempah, SimonBaeumer

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