Add notes to the Sidekiq and Container Registry documentation

What does this MR do?

This MR adds notes to the documentation for Sidekiq and the Container Registry regarding additional configuration needed when Sidekiq and the Container Registry are installed on different nodes. In an on-premise install, we found that the Sidekiq jobs related to cleaning up container registry repository tags were failing because:

1. The registry.key file was missing from the Sidekiq nodes, and
2. There was no registry URL defined so localhost:5000 was being used

I couldn't find anywhere in the documentation where this information was stated, so I thought I would submit this MR to help others who might run into this issue in the future.