Add import and export settings page to global nav

What does this MR do and why?

Add global navigation item for content moved to new page in MR: gitlab!129584 (merged).

Screenshots, screen recordings, or links to review app

• Before: https://docs.gitlab.com/ee/administration/settings/import_and_export_settings.html
• After: https://eread-add-import-and-export-settings-page-to-global-nav.docs.gitlab-review.app/ee/administration/settings/import_and_export_settings.html

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.