index: add node.js section and express gcp backend

Front page link for #1050

Blocking on https://github.com/GoogleCloudPlatform/gcloud-node-todos/pull/19 and a deploy on gcloud-todos.appspot.com

/cc @viviancromwell @stephenplusplus @s3ththompson 
I don't think the homepage should target one specific framework + stack. If a user wants to see Angular + ____, they can go to todomvc.com/examples/angularjs and if there's a backend implementation, a link will appear in the sidebar to the source and demo.

Then, my concern with "Express + Stack" is, what do we do when there are more than one of these exact pairings, with different things used behind them. Should we categorize:

Node.js > Express > gcloud-node
Node.js > Express > MongoDB

I'll defer to @addyosmani for how he wants to handle this :+1: