Improves empty state for dependency proxy

What does this MR do and why?

Improves empty state for dependency proxy

Adds example & link to documentation with copyable example.

Screenshots or screen recordings

Before	After
	Screen_Recording_2023-06-29_at_11.37.55_pm

How to set up and validate locally

1. Enable dependency proxy on GDK.
2. Visit Group > Packages and registries > Dependency proxy
3. Should be able to see updated empty state
4. Assuming 172.16.123.1:3000/gitlab-org/dependency_proxy/containers is the Dependency proxy image prefix on the page, run the following on the terminal:
   • docker pull 172.16.123.1:3000/gitlab-org/dependency_proxy/containers/hello-world@sha256:995efde2e81b21d1ea7066aa77a59298a62a9e9fbb4b77f36c189774ec9b1089
   • docker pull 172.16.123.1:3000/gitlab-org/dependency_proxy/containers/hello-world
5. Empty state should not be visible anymore.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Closes #343968 (closed)