added article to toc

Author: MashaMSFT

articles/sql-database/toc.yml

@@ -740,6 +740,8 @@
         href: sql-database-managed-instance-configure-vm.md
       - name: Point-to-site connection
         href: sql-database-managed-instance-configure-p2s.md
+      - name: Long-term backup retention
+        href: sql-database-managed-instance-long-term-backup-retention-configure.md
     - name: Load data
       items:
       - name: Restore database backup