There aren't any official documents about how to properly configure assisted to use a mirror registry for a disconnected CIM install.

Good starting point: https://github.com/jparrill/ztp-the-hard-way/blob/main/docs/disconnected-ZTP-flow-hub-deployment.md

Related slack threads:

https://redhat-internal.slack.com/archives/CUPJTHQ5P/p1689958118204999

https://redhat-internal.slack.com/archives/CUPJTHQ5P/p1690235684421209

Potentially this documentation would live here https://docs.openshift.com/container-platform/4.13/scalability_and_performance/ztp_far_edge/ztp-preparing-the-hub-cluster.html#ztp-configuring-the-cluster-for-a-disconnected-environment_ztp-preparing-the-hub-cluster