docs(storybook): hide gen2 code blocks by default (#6481)

- keep docs pages focused on rendered examples unless readers expand source
- document that story-level sourceState overrides still work

Author: rubencarvalho

2nd-gen/packages/swc/.storybook/preview.ts

@@ -187,7 +187,8 @@ const preview = {
       canvas: {
         withToolbar: true,
         layout: 'centered',
-        sourceState: 'shown',
+        // Individual stories can override this with parameters.docs.canvas.sourceState.
+        sourceState: 'hidden',
       },
       source: {
         excludeDecorators: true,