Create demo for static Linked Dropdowns

[demianrenzulli]

The current demo for Linked Drowpdowns makes use of nested amp-list component, that issue server-side requests to populate the main and secondary drop-downs.

Some sites count with this information at load time, and would prefer to have a static implementation, that doesn't require having an endpoint.

For this, we can use amp-bind, to show/hide certain options on the second drop-down, when an option on the first one is selected.