Docs: GitLab Dependency Proxy only support Docker

What does this MR do?

Change the documentation to explain that GitLab Dependency Proxy only support Docker.

See also: https://docs.gitlab.com/ee/user/packages/dependency_proxy/

For other package managers you have another feature called "Package registry", which is not the same thing. Now the admin documentation says that Depedency Proxy is used for a variety of package managers, which is just incorrect. And at best very confusing (for me it was!).

GitLab Dependency Proxy only support Docker. See also user documentation: https://docs.gitlab.com/ee/user/packages/dependency_proxy/. For other package managers you have another feature called "Package registry", which is not the same thing.

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
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Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel. I selected: @rdickenson.

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