[docs] change code samples to import by name with inline script

I would suggest importing by name with inline script instead and let the tools will handle the resolution:

<script type=module>
import 'package-name/existing-element.js';
</script>

This is what we did for PolymerElement demos (e.g. https://github.com/PolymerElements/iron-list/blob/master/demo/index.html#L20).

Originally posted by @keanulee in #263

[justinfagnani]

This appears to be done in the current site.