document add rest schema (https://github.com/woocommerce/woocommerce-blocks/pull/6960)

plugins/woocommerce-blocks/docs/third-party-developers/extensibility/rest-api/available-endpoints-to-extend.md

@@ -2,18 +2,34 @@
 
 ## Table of Contents <!-- omit in toc -->
 
--   [`wc/store/cart`](#wcstorecart)
-    -   [Passed Parameters:](#passed-parameters)
-    -   [Key:](#key)
--   [`wc/store/cart/items`](#wcstorecartitems)
-    -   [Passed Parameters:](#passed-parameters-1)
-    -   [Key:](#key-1)
--   [`wc/store/products`](#wcstoreproducts)
-    -   [Passed Parameters:](#passed-parameters-2)
-    -   [Key:](#key-2)
+- [`wc/store/checkout`](#wcstorecheckout)
+  - [Passed Parameters](#passed-parameters)
+  - [Key](#key)
+- [`wc/store/cart`](#wcstorecart)
+  - [Passed Parameters](#passed-parameters-1)
+  - [Key](#key-1)
+- [`wc/store/cart/items`](#wcstorecartitems)
+  - [Passed Parameters](#passed-parameters-2)
+  - [Key](#key-2)
+- [`wc/store/products`](#wcstoreproducts)
+  - [Passed Parameters](#passed-parameters-3)
+  - [Key](#key-3)
 
 To see how to add your data to Store API using ExtendSchema, [check this document](./extend-rest-api-add-data.md). This is a list of available endpoints that you can extend. If you want to add a new endpoint, [check this document](./extend-rest-api-new-endpoint.md).
 
+## `wc/store/checkout`
+
+The cartcheckout endpoint is extensible via ExtendSchema. The data is available via the `extensions` key in the response.
+
+### Passed Parameters
+
+-   `data_callback`: none.
+-   `schema_callback`: none.
+
+### Key
+
+-   `CheckoutSchema::IDENTIFIER`
+
 ## `wc/store/cart`
 
 The main cart endpoint is extensible via ExtendSchema. The data is available via the `extensions` key in the response.