added migration content

Author: MashaMSFT

docs/sql-server/index.yml

@@ -34,6 +34,8 @@ landingContent:
     linkLists:
     - linkListType: overview
       links:
+      - text: Migration content
+        url: sql-server/migrate/index.yml
       - text: Data Migration Guide
         url:  https://datamigration.microsoft.com/
       - text: Database Compatibility Certification