message: Add code preview block to interaction example
diff: The app that rendered checkbox and handles the `onSelectionEnter`, `onSelectionLeave` events.
  +Demo:
  +
  <p align="center"><img src="https://media.giphy.com/media/j2Rh9Ww5nKvwowB5gi/giphy.gif" width="480"></p>
  +Code example:
  +
  +```javascript
  +const [checked, setChecked] = React.useState(false);
  +...
  +<Frame style={styles.frame}>
  + <Checkbox
  + checked={checked}
  + label="select me"
  + onSelectionEnter={() => setChecked(true)}
  + onSelectionLeave={() => setChecked(false)}
  + />
  + <Text style={styles.text}>{checked ? 'Checkbox selected' : 'Checkbox not selected'}</Text>
  + <Text style={styles.text}>{`Counter: ${counter}`}</Text>
  +</Frame>
  +```
  +
  [How to run](../../contributing.md#running-examples)
  