Docs: how the global nav works

What does this MR do?

Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23065 to EE.

Related issues

• https://gitlab.com/gitlab-com/gitlab-docs/issues/169
• gitlab-com&21 (closed)
• https://gitlab.com/gitlab-com/gitlab-docs/issues/260

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
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23065

Review checklist

• Your team's review (required)
• Merge this EE-MR first, CE-MR afterwards (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23065)