Document the cart to platform-checkout handoff with Spark as reference

Problem: The weakest documented seam in the theme docs is the one merchants care about most: how a theme cart hands off to platform checkout. There is no page covering the handoff mechanics, no worked side-cart example, and the variant-selection JS reference points to an unlinked "Base Theme."

Evidence: content/docs/storefront/themes/templates/urls-and-template-paths.mdx (checkout template named, no handoff topic); content/docs/storefront/graphql/index.mdx (side carts called the most common use, no worked example); content/docs/storefront/themes/guides/product-variants.mdx (defers to an unlinked base theme).

Done when:

• A handoff page documents the cart-to-checkout transition: trigger, URL, and how cart state carries into /checkout/.
• A side-cart tutorial exists with Spark's implementation linked as the public reference.
• The variant guide links Spark's SparkVariantState as the reference implementation.
• An outside developer can trace add-to-cart through checkout handoff from docs alone.

Start with content/docs/storefront/themes/templates/urls-and-template-paths.mdx, content/docs/storefront/graphql/index.mdx, and content/docs/storefront/themes/guides/product-variants.mdx, then inspect Spark's checkout, side-cart, and SparkVariantState references. Done means the handoff trigger, URL, cart-state behavior, side-cart example, and variant reference are linked clearly enough to trace add-to-cart through checkout from the docs alone.