explain processor arch options on container images page

[dmuelle]

on https://openliberty.io/docs/latest/container-images.html, need to add explanation of the different processor architectures that are supported. These options are visible in icr.io, but not dockerhub. we don't need to document the specific arch notations in those tags because the manifest maps the image to the correct arch. Just document what is supported.

ibmjava type images support amd64, ppc64le, s390x
beta image for OL on icr.io supports amd64
All other images support amd64, ppc64le, s390x, arm64

Linux® x86_64 (amd64), Linux on Power® (ppc64le), Linux on IBM Z (s390x), Linux on ARM (arm64)