Add docs for Dependency Proxy Docker Hub auth

What does this MR do and why?

We added the API in Allow private registry connections with the dep... (#331741 - closed) and the UI in Expose Dependency Proxy auth settings for Docke... (#521954 - closed)

This MR updates the documentation added in Release the Docker Hub credentials support in t... (!183020 - merged)

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Related to #521954 (closed)