Demo checkout page

### Description

Hello
I would like to check how you guys worked with AMP on checkout page but found only 'not yet implemented' -> http://prntscr.com/1qy768k

The main reason why I want to check how you did it because it is hard for me to make full compatible my own amp project based on your scripts with WooCommerce checkout ( main issue with countries and states that should load with dependence )

Regards

### Reproduction Steps

https://camp.samples.amp.dev/checkout.html - your example ecommerce project

### Relevant Logs

_No response_

### Browser(s) Affected

_No response_

### OS(s) Affected

_No response_

### Device(s) Affected

_No response_

### AMP Version Affected

_No response_

Start with the example ecommerce checkout at https://camp.samples.amp.dev/checkout.html and inspect the current “not yet implemented” state. The issue describes a need for an AMP-compatible checkout, including dependent country and state loading, but names no source files or tests. Done would require a documented or implemented checkout example that addresses the requested behavior.

Written by the indexing model from the issue text.