Docs: redirect old Maven Repo docs to new ones

What does this MR do?

• Adds back old pages for renaming the feature to Maven Repository
• Redirecting the pages will un-index them from Google search
• Add product badge to admin doc

Related issues

• https://gitlab.com/gitlab-org/gitlab-ee/commit/cb7b2a6e354c76c3b717829c17322a8b872e2a1a
• https://gitlab.slack.com/archives/C9CTGNZDJ/p1537540511000100

Author's checklist

• Apply the correct labels and milestone
• Correctly apply the product badges and tiers

Review checklist

• Technical writer's review (required)

cc/ @joshlambert @axil FYI ;)