DEVDOCS-6593 - Add buybutton and mcp as valid checkout urls (#1149)

<!-- Ticket number or summary of work -->
# [DEVDOCS-6593]


## What changed?
<!-- Provide a bulleted list in the present tense -->
* added buybutton and mcp as accepted checkout urls

## Release notes draft
Since the [Checkout
URL](https://developer.bigcommerce.com/docs/storefront/cart-checkout/checkout-url)
launched in July/August 2025, it has been widely adopted by other
Commerce teams. To better accommodate these teams and their projects, we
have expanded the values available for the order_source parameter to
include buybutton and mcp.

ping @bigcommerce/dev-docs @bc-terra 


[DEVDOCS-6593]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6593?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: chris-nowicki

docs/storefront/cart-checkout/checkout-url.mdx

@@ -28,6 +28,8 @@ https://{store-url}/checkout/?products={id}:{quantity},{id}:{quantity}&coupon={c
   * facebookshop  
   * facebookmarketplace  
   * www
+  * buybutton
+  * mcp
 
 ## Examples