[enterprise-4.12] Move scattered postinstall content to image config section

[abrennan89]

Manual cherrypick of 133fa71 xref: #79596

[ocpdocs-previewbot]

🤖 Thu Aug 01 20:12:27 - Prow CI generated the docs preview:

https://79891--ocpdocs-pr.netlify.app/
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/openshift_images/image-configuration.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/openshift_images/managing_images/using-image-pull-secrets.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/post_installation_configuration/cluster-tasks.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/post_installation_configuration/post-install-image-config.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/post_installation_configuration/preparing-for-users.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/registry/configuring-registry-operator.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/support/remote_health_monitoring/opting-out-of-remote-health-reporting.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/updating/updating-restricted-network-cluster/restricted-network-update.html
https://79891--ocpdocs-pr.netlify.app/openshift-enterprise/latest/updating/updating-restricted-network-cluster/restricted-network-update-osus.html

[openshift-ci]

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