docs: improve payment related guides (#12502)

* improve guide

* update guides

* small change

www/apps/resources/sidebars/payment.mjs

@@ -42,6 +42,11 @@ export const paymentSidebar = [
             path: "/commerce-modules/payment/payment-provider",
             title: "Payment Module Provider",
           },
+          {
+            type: "link",
+            path: "/commerce-modules/payment/payment-checkout-flow",
+            title: "Payment in Checkout",
+          },
           {
             type: "link",
             path: "/commerce-modules/payment/account-holder",