Docs route maps

What does this MR do?

• Move docs on Route Maps from ci/environments.md to ci/review_apps/index.md with a more explicit heading for SEO
• Crosslink Route Maps with CI environments and merge requests

Related issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53178

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from other subject-related docs

Review checklist

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