Improve Registry README

[alexandrevicenzi]

The Registry README is rather basic and it took me a while to figure out that it is in fact Distribution Registry under the hood.

The description should include a link to the documentation at the very least. We should also instruct how to push and pull from a local registry, it might be obvious for many, but not for everyone.

[alexandrevicenzi]

Also, are we married to this name and short description?

SLE OCI Container Registry (Distribution) Container Image
OCI Container Registry (Distribution) container based on the SLE Base Container Image.

IMHO, the version below (or a similar one) is more concise and in line with other images.

SLE Distribution Registry
Distribution Registry is an OCI Container Registry based on the SLE Base Container Image.

[alexandrevicenzi]

It seems that the path /etc/docker/registry/config.yml is also wrong and should be /etc/registry/config.yml.