Storefront API build guide

A new end-to-end guide covers building and shipping a custom storefront with the Storefront API.

Added an end-to-end Storefront API guide for agents and frontend developers
building a custom storefront. The guide covers the direct frontend
architecture, setup checklist, fetch helpers, catalog flow, guest and customer
cart checkout, customer auth, locations, password reset, payment rendering,
subscriptions, course access, and the final implementation checklist.