Adding "Getting Started" information to the index page

[LucaJunge]

Hello,

I am currently learning WebGL2 with the help of the website, but felt the index page was missing a starting point for beginners. As the "Fundamentals" articles requires having a setup and the boilerplate code (the function webglUtils.resizeCanvasToDisplaySize is mentioned in there without a previous mention of this boilerplate code and can confuse users where it comes from) it would be good to show at least the order of the 3 articles.

This PR adds a new section to the index page with more information on where to start.

Previously

With Getting Started