magento#589: Add a tutorial for the checkout workflow

Author: atwixfirster

guides/v2.3/graphql/tutorials/checkout/checkout-billing-address.md

@@ -212,26 +212,41 @@ mutation {
 
 ### Use the existing customer address
 
+Use a query below to retrieve the list of customer addresses:
+
 **Request**
+
 ```text
-mutation {
-  setBillingAddressOnCart(
-    input: {
-      cart_id: "{{ CART_ID }}"
-      billing_address: {
-          customer_address_id: {{ CUSTOMER_ADDRESS_ID }}
-      }
+query {
+  customer {
+    addresses {
+      id
+      default_billing
+      default_shipping
     }
-  ) {
-    cart {
-      billing_address {
-        city
-      }
+  }
+}
+```
+
+**Response**
+
+```text
+  "data": {
+    "customer": {
+      "addresses": [
+        {
+          "id": 2,
+          "default_billing": true,
+          "default_shipping": true
+        }
+      ]
     }
   }
 }
 ```
 
+Define `customer_address_id` to assign the existing customer address. 
+
 **Request**
 
 ```text
@@ -240,19 +255,7 @@ mutation {
     input: {
       cart_id: "{{ CART_ID }}"
       billing_address: {
-         address: {
-          firstname: "John"
-                lastname: "Doe"
-                company: "Company Name"
-                street: ["320 N Crescent Dr", "Beverly Hills"]
-                city: "Los Angeles"
-                region: "CA"
-                postcode: "90210"
-                country_code: "US"
-                telephone: "123-456-0000"
-                save_in_address_book: false
-         }
-#         use_for_shipping: true
+          customer_address_id: {{ CUSTOMER_ADDRESS_ID }}
       }
     }
   ) {
@@ -271,26 +274,17 @@ mutation {
         }
         address_type
       }
-      shipping_addresses {
-        firstname
-        lastname
-        company
-        street
-        city
-        postcode
-        telephone
-        country {
-          code
-          label
-        }
-        address_type
-      }
     }
   }
 }
 ```
 
+where 
+`{{ CART_ID }}` - shopping cart unique ID from [Step 2. Create empty cart]({{ page.baseurl }}/graphql/tutorials/checkout/checkout-add-product-to-cart.html).
+`{{ CUSTOMER_ADDRESS_ID }}` - customer address ID (value from `entity_id` field of `customer_address_entity` table).
+
 **Response**
+
 ```json
 {
   "data": {
@@ -312,26 +306,7 @@ mutation {
             "label": "US"
           },
           "address_type": "BILLING"
-        },
-        "shipping_addresses": [
-          {
-            "firstname": "John",
-            "lastname": "Doe",
-            "company": "Company Name",
-            "street": [
-              "320 N Crescent Dr",
-              "Beverly Hills"
-            ],
-            "city": "Los Angeles",
-            "postcode": "90210",
-            "telephone": "123-456-0000",
-            "country": {
-              "code": "US",
-              "label": "US"
-            },
-            "address_type": "SHIPPING"
-          }
-        ]
+        }
       }
     }
   }