Add "Add To User List" button on product page

## 📖 Missing Documentation for Add to List on product page

### Problem
On the [example product page](https://amp.dev/documentation/examples/e-commerce/product_page/?format=websites) there's lots of examples for how to implement different features. It would be great to have an example on how to implement a "Add To User List" button where the user needs to be logged in in order to add to the list, probably through amp-access and than it needs to check if the user has previously added it or not in order to show a different icon or text.

There is the [favorite button example](https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/) which is kind of similar to this but is not user account related.

### Location
Should be added to either of the following pages

- [example product page](https://amp.dev/documentation/examples/e-commerce/product_page/?format=websites)
- [example favorite button](https://amp.dev/documentation/examples/interactivity-dynamic-content/favorite_button/)

### Content type
Similar to above mentioned pages and should probably be added as a new section.

### Information architecture (optional)
In our case we've had difficulty using the amp-access from within the amp-list template, where the amp-access attributes were silently stripped away. The doc should clearly state what is possible and what is not and in which cases there's a workaround to get something like this working.

Start with the example product page and favorite button example linked in the issue, then compare their existing sections and amp-access guidance. Document an Add To User List example covering login requirements, previously added state, and the amp-access and amp-list limitation or workaround described in the issue.

Written by the indexing model from the issue text.