[DOCS] Update documentation regarding bootc image mode

[cdrage]

Was not able to find a "documentation" template.. sorry!

Issue

https://github.com/openshift/microshift/blob/main/docs/contributor/image_mode.md#create-virtual-machine talks about downloading an .ISO and using kickstart.

However, this goes against the standard documentation with regards to building bootc images as per RHEL docs: https://docs.redhat.com/en/documentation/red_hat_enterprise_linux/9/html/using_image_mode_for_rhel_to_build_deploy_and_manage_operating_systems/creating-bootc-compatible-base-disk-images-with-bootc-image-builder_using-image-mode-for-rhel-to-build-deploy-and-manage-operating-systems

Solution

IMHO, I believe it should be using bootc-image-builder to build microshift rather than using kickstart. There should also be a method somehow to be able to add the openshift secret during build time.