Add documentation for Private Registry

[r0ckarong]

Describe your changes

Adds docs for SUSE Private (Container) Registry

Content from: https://confluence.suse.com/display/ENGCTNRSTORY/Documentation+-+SUSE+Private+Registry+Powered+by+Harbor+2.1

Creates two new documents: Private Registry Documentation (Introduction, Deployment, Administration) Private Registry User Guide

Related Issues / Projects

Please provide links to Bugzilla and other GitHub projects with your description if they are related to the changes.

Relates to: https://github.com/SUSE/avant-garde/issues/1945

[dirkmueller]

@r0ckarong please use the documentation from https://confluence.suse.com/display/ENGCTNRSTORY/Documentation+-+SUSE+Private+Registry+Powered+by+Harbor+2.1 as source

[r0ckarong]

This will now be done as a best practice document.

https://github.com/SUSE/suse-best-practices