Move `packages/demos-react/site` to `demos/react/`

[appsforartists]

We expect to have many different demos packages:

• demos-react
• demos-cycle
• demos-ng2
• demos-dom

each with its own devServer and build. Running build in any package should generate a new file here:

/demos/${ framework name }/bundle.js

That lets us host each framework from the same demos site, without having them depend on one another.

---

In demos/README.md:

The Material Motion Demos site

This folder contains only the HTML scaffolding for https://material-motion-demos.firebaseapp.com/.

The JavaScript source is kept in these packages:

• DOM (no framework)
• Angular
• Cycle
• React