improve example page

I think the example page is too old and not convenient enough for learners to use.

What if we try to make pages like this: https://www.threejs.org.cn/gallery/
We can just make a simple online code editor, when people click a demo, we don’t need to load the page in the iframe, we can ajax the code to the online editor and editor run the code. It‘’s a more convenient way to use. (And in this link, I use ACE as a code editor and also provide common used three.js’ key words hints)