Make layout examples dynamic and improve route metadata with Express Map

This removes all the specific routes for each layout example and replaces them with one, dynamic route: `/layouts/:layout/`.

This also updates how the data for the nav menu is generated, and adds a new `pathTo` Handlebars helper for writing URLs to routes, including the new dynamic layouts route:

```html
<a href="{{pathTo 'home'}}">Home</a>
<a href="{{pathTo 'layout' layout='pricing'}}">Pricing Layout</a>
```