Add details on how to build an AMI image for use with the Terraform

The terraform currently refers to an AMI image, but does not say how to build one if a user wishes to use their own (or doesn't have access to the AMI. Add documentation on how to create a RHEL8 UBI image with STIGs and RKE2 as an example of creating a base image and distro to use with the infrastructure as code.

Use the RKE2 Image Builder as a reference guide.