Add new project import/export troubleshooting nav item

What does this MR do and why?

Adds navigation for new topic added in: gitlab!103297 (merged).

Also creates a hierarchy for related topics.

• Before: https://docs.gitlab.com/ee/user/project/settings/import_export.html
• After: https://eread-add-new-project-import-export-troubleshooting-nav-item.docs.gitlab-review.app/ee/user/project/settings/import_export.html
• New topic (before): https://docs.gitlab.com/ee/user/project/settings/import_export_troubleshooting.html
• New topic (after): https://eread-add-new-project-import-export-troubleshooting-nav-item.docs.gitlab-review.app/ee/user/project/settings/import_export_troubleshooting.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.