docs: added mouse position example

Author: logaretm

docs/guide/mouse-position.md

@@ -52,4 +52,8 @@ export default {
 </script>
 ```
 
-TODO: EXAMPLE
+### Demo
+
+<iframe src="https://codesandbox.io/embed/vue-use-web-use-scroll-position-jb5sz?fontsize=14&module=%2Fsrc%2FApp.vue" title="vue-use-web: use mouse position" allow="geolocation; microphone; camera; midi; vr; accelerometer; gyroscope; payment; ambient-light-sensor; encrypted-media; usb" style="width:100%; height:500px; border:0; border-radius: 4px; overflow:hidden;" sandbox="allow-modals allow-forms allow-popups allow-scripts allow-same-origin"></iframe>
+
+> Credits: [Codepen by Chuloo](https://codepen.io/Chuloo/pen/RQYbvm)