docs(checkout): cross-link cart_id from Create Checkout

[matthutchinson]

What

Add a one-line cross-link from the Create Checkout section in checkout.md to the existing Cart-to-Checkout Conversion section in cart.md, so a reader on the checkout page can discover cart_id and the conversion contract.

Why

The cart capability extends create_checkout with a cart_id field (defined in cart.json#/$defs/checkout and covered in prose under Cart → Cart-to-Checkout Conversion). The cart page covers it well, but the checkout page has no signpost — a reader on /specification/checkout/#create-checkout cannot tell that this field exists or where to find it.

This adds a single discoverability hop. No content duplication.

Change

 ### Create Checkout

 ...
 **Recommendation**: To minimize discrepancies and a streamlined user experience,
 product data (price/title etc.) provided by the business through the feeds
 **SHOULD** match the actual attributes returned in the response.

+When the [Cart](cart.md) capability is negotiated, the request payload
+accepts an additional `cart_id` field for cart-to-checkout conversion. See
+[Cart → Cart-to-Checkout Conversion](cart.md#cart-to-checkout-conversion) for
+the field contract.
+
 {{ method_fields('create_checkout', 'rest.openapi.json', 'checkout') }}

Verification

Local mkdocs build --strict succeeds with no non-cairosvg warnings. The cross-link resolves to ../cart/#cart-to-checkout-conversion, which is a stable existing anchor.

---

[damaz91]

While I see the value of this PR from a clarity standpoint, I also think we either do this for all extensions or not doing this at all. If we approve this PR we should also add a callout for every Checkout extension (Fulfillment, Discount, Buyer Consent, ..)