Refactor the Packages documentation

What does this MR do?

Refactors the docs introduced in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8673/diffs.

The info presented in doc/administration/maven_repository.md and doc/administration/npm_registry.md was almost identical and was basically pointing to doc/administration/packages.md, so I redirected those pages to the latter and added all needed info in the new doc.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers

Review checklist

• PM's review (recommended, but not a blocker)
• Technical writer's review (required)