docs(cdk/drag-drop): scrollable example not showing (#31627)

Fixes that the scrollable example wasn't showing up properly, because it wasn't exported from the index file.

Fixes #31619.

(cherry picked from commit 84aea87)

Author: crisbeto

src/components-examples/cdk/drag-drop/index.ts

@@ -19,3 +19,4 @@ export {CdkDragDropTableExample} from './cdk-drag-drop-table/cdk-drag-drop-table
 export {CdkDragDropMixedSortingExample} from './cdk-drag-drop-mixed-sorting/cdk-drag-drop-mixed-sorting-example';
 export {CdkDragDropTabsExample} from './cdk-drag-drop-tabs/cdk-drag-drop-tabs-example';
 export {CdkDragDropCopyListExample} from './cdk-drag-drop-copy-list/cdk-drag-drop-copy-list-example';
+export {CdkDragDropScrollableExample} from './cdk-drag-drop-scrollable/cdk-drag-drop-scrollable-example';