Reviews storefront: three more sort options

The V3 storefront review endpoints accept 8 sort values. We listed 5. The full list: smart, smart_optimistic, date, rating, images, votes_up, votes_down and badge.

Also clarified:

• sortings on the search endpoints takes the same 8 values. It's applied first, and the sort query parameter then breaks ties. An unsupported sortBy value returns 500, so send only the listed values.
• perPage is capped at 150 with no error. pagination.perPage in the response shows the value used.

Updated guides: Building custom reviews widgets and Headless integration.