Add documentation for dependency proxy feature

What does this MR do?

Docs for https://gitlab.com/gitlab-org/gitlab-ee/issues/7934

Links

• Issue - https://gitlab.com/gitlab-org/gitlab-ee/issues/7934
• Backend - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10676, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10681, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9750
• UI - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10386
• Docs - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12210
• Config - https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10675, omnibus-gitlab!3206 (merged)