Docs site architecture / how the global nav works

What does this MR do?

• Adds a document for the docs site architecture - REVIEW APP
• Documents how the global navigation works - REVIEW APP
• Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/260

Related issues

• https://gitlab.com/gitlab-com/gitlab-docs/issues/169
• gitlab-com&21 (closed)
• https://gitlab.com/gitlab-com/gitlab-docs/issues/245

Author's checklist

• Apply the correct labels and milestone
• Port the MR to EE (or backport from CE): https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8771

Review checklist

• Your team's review (required)
• Merge the EE-MR first (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8771), CE-MR afterwards