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 
was mainly think of that eventually there will be other cloud providers who participate and have the similar implementation for (fe and be but different datastore/sql etc), how do we plan to distinguish that?