update missing doc links

doc/api/projects.md

@@ -1326,6 +1326,10 @@ POST /projects/:id/housekeeping
 
 Read more in the [Branches](branches.md) documentation.
 
+## Project Import/Export
+
+Read more in the [Project export](project_import_export.md) documentation.
+
 ## Project members
 
 Read more in the [Project members](members.md) documentation.

doc/user/project/settings/import_export.md

@@ -22,6 +22,7 @@
 >    in the import side is required to map the users, based on email or username.
 >    Otherwise, a supplementary comment is left to mention the original author and
 >    the MRs, notes or issues will be owned by the importer.
+>  - Control project Import/Export with the [API](../../../api/project_import_export.md).
 
 Existing projects running on any GitLab instance or GitLab.com can be exported
 with all their related data and be moved into a new GitLab instance.