Add focus example

Author: pomber

packages/mdx/dev/content/rows.mdx

@@ -20,6 +20,20 @@ console.log(7)
 
 </CH.Code>
 
+<CH.Code rows="focus" showCopyButton={false} lineNumbers >
+
+```js focus=3:4,6[1:11]
+console.log(1)
+console.log(2)
+console.log(3)
+console.log(4)
+console.log(5)
+console.log(6)
+console.log(7)
+```
+
+</CH.Code>
+
 <CH.Code rows={4} style={{width: 200 }} lineNumbers >
 
 ```js