Update codepen template

[teodragovic]

On demos and examples link to codepen template opens broken pen.

I think the template can be rewritten to use functional component and import via skypack direct in pen (makes it a bit more obvious where preact comes from vs using codepen settings)

minimal example:

import { h, render, Fragment } from 'https://cdn.skypack.dev/preact';
import { useState } from 'https://cdn.skypack.dev/preact/hooks';

/** @jsx h */

const App = () => {
   const [ counter, setCounter  ] = useState(1);
   
   return (
      <div>Hello world</div>
   );
}; 

render(<App />, document.body);