docs(cdk/scrolling): make ScrollingModule the primary export (#20112) (#20195)

This designates ScrollingModule as the primary module for cdk/scrolling.

Fixes #20112

Author: howardjing

src/cdk/scrolling/scrolling-module.ts

@@ -20,6 +20,9 @@ import {CdkVirtualScrollViewport} from './virtual-scroll-viewport';
 })
 export class CdkScrollableModule {}
 
+/**
+ * @docs-primary-export
+ */
 @NgModule({
   imports: [
     BidiModule,