Register Notion page in sidebars.js

The Data integrations category in sidebars.js uses an explicit `items`
array (not autogenerated), so the new page needs to be listed there to
show up in the left-hand nav. Same fix pattern as PR #6141 commit bcfd15b.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Alex Francoeur

sidebars.js

@@ -1256,6 +1256,7 @@ const sidebars = {
           },
           items: [
             'integrations/tools/data-integration/middleware/index',
+            'integrations/tools/data-integration/notion/index',
             'integrations/tools/data-integration/retool/index',
             'integrations/tools/data-integration/easypanel/index',
             'integrations/tools/data-integration/splunk/index',