docs: document skip_purchase_success in web purchase links (#960)

Author: nicfix

docs/web/web-billing/web-purchase-links.mdx

@@ -179,3 +179,7 @@ Allows you to pre-select a package by referencing a RevenueCat `package_id` for
 #### Custom metadata
 
 See [custom metadata](/web/web-billing/custom-metadata)
+
+#### Skip purchase success
+
+Add `skip_purchase_success=true` to bypass the final "Purchase Complete" step and immediately trigger your configured success behavior (success page or custom redirect).