update toc

Author: stevestein

docs/sql-operations-studio/TOC.md

@@ -13,6 +13,7 @@
 ## [Backup and restore](tutorial-backup-restore-sql-server.md)
 # How-to guides
 ## [Dashboards & insight widgets](insight-widgets.md)
+## [Extensions](extensions.md)
 ## [Code snippets](code-snippets.md)
 ## [Integrated terminal (PowerShell, Bash)](integrated-terminal.md)
 ## [Keyboard shortcuts](keyboard-shortcuts.md)