add: left menu examples

[eviltik]

Let create a dynamic left menu in js/css so we have a way to quickly switch between examples. Like threejs examples left menu without pictures (puppeteer is a way to automate screenshot and make thumbail, but it should be an improvement for later).

Examples will be defined in the js file, something like this ;

const exampleList = [
{
    title: 'Buttons - Panel',
    url:'../buttons/'
}