Add import-related developer topics to global navigation

What does this MR do and why?

Improve navagability of the ~"group::import" developer documentation by adding some previously-missing topics to the global navigation.

Screenshots, screen recordings, or links to review app

• Before: https://docs.gitlab.com/ee/development/import_export.html
• After: https://eread-add-import-related-developer-topics-to-global-nav.docs.gitlab-review.app/ee/development/import_export.html

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• Before: https://docs.gitlab.com/ee/development/import_project.html
• After: https://eread-add-import-related-developer-topics-to-global-nav.docs.gitlab-review.app/ee/development/import_project.html

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• Before: https://docs.gitlab.com/ee/development/github_importer.html
• After: https://eread-add-import-related-developer-topics-to-global-nav.docs.gitlab-review.app/ee/development/github_importer.html

Merge request acceptance checklist

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