Document changing provider registries

Add guide for safely changing providers between compatible packages in
different OCI registries (e.g., upbound → crossplane-contrib).

The guide focuses on the manual coordination approach needed when
providers have family dependencies that create ownership conflicts.
Includes step-by-step process, troubleshooting, and rollback procedures.

Also adds cross-reference from the upgrade-to-v2 guide for users
switching registries during their v2 upgrade.

Signed-off-by: Nic Cope <nicc@rk0n.org>

Author: negz