Add documentation for product identifier schema

Summary

Technical choices that affect more than this issue:

Adapted the other API sentences, hopefully improving the flow of them
Fixed the indentation of the Alter the `Product` piece. in the Yoast WooCommerce SEO page

This PR can be acceptance tested by following these steps:

Go to /features/schema/pieces/product/#optional-properties
Verify the add product identifiers
Scroll down to the API section (/features/schema/pieces/product/#api)
Verify the product API section makes sense (maybe it the code snippet works)
Go to /features/schema/pieces/offer/#api
Verify the offer API section makes sense (maybe it the code snippet works)
Go to the Yoast WooCommerce SEO page' filter section (/features/schema/plugins/woocommerce-seo/#filters)
Verify the text makes sense and the links go to the previous API sections
Scroll down to the more information section of the Yoast WooCommerce SEO page (/features/schema/plugins/woocommerce-seo/#more-information)
Verify the ProductGroup schema piece page is in there
Verify the link works
Verify all the content on the ProductGroup schema piece page

[ ] Security - I have thought about any security implications this code might add.
[ ] Performance - I have checked that this code doesn't impact performance (greatly).
[ ] Caching - I have analyzed the caching methods that this code touches and have added instructions to deal with those.
[ ] Tested - I have tested this code to the best of my abilities.
[ ] Automated tests - I have added unit tests to verify the code works as intended.
[ ] Testability - I have added unique ids to elements, so they can be located in automated testing.
[ ] I have altered a filename.
- [ ] I have adjusted the ID property accordingly and updated all internal links.
- [ ] I have added the redirect to the _redirects file in the root of the project.