Add orphaned API topics to global nav

This MR adds two orphaned API doc topics to the global navigation.

• https://docs.gitlab.com/ee/api/resource_groups.html

• https://docs.gitlab.com/ee/api/project_relations_export.html

• Link to the merge request of the new page:

• Give the MR a descriptive title.

• Link to the merge request that introduced the new page.