Homepage for https://registry.gitlab.com/

As a user, if I connect to https://registry.gitlab.com/ I just see a blank page, but we can improve this to show something useful.

Possible proposals for this could be:

help message that briefly tells you what is that domain for, with possibly links to documentation in how to use it
registry catalog, similar to https://hub.docker.com/: it can be used to share and publish public images outside GitLab

First one could be simple, second one is part of a possible bigger plan on how to leverage internal GitLab container registry.