Document multi-arch fields and recommendations

[yosifkit]

Document how to setup the library file for multi-arch (all the fields added in #3009).

Add recommendations on how to setup a base image for multi-arch (would love to push multi-stage builds, but some arches don't have a new enough docker version 😢).

Recommendations on how to setup a normal (non-base image) repo. For example do not create architecture specific Dockerfiles, try to make a single Dockerfile that works for all architectures; the only difference will be the base image that is pulled in via manifest lists. Currently the Jenkins jobs from https://github.com/docker-library/oi-janky-groovy will work around the non-existent manifest lists by pulling form the arch specific prefix and tagging it locally without the prefix, so that we don't have to sed the Dockerfile and can just use bashbrew build directly.