Added documentation for payment methods API (#7369)

Co-authored-by: jessy <[email protected]>

changelog/add-7368-add-api-documentation-for-payment-methods-api

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+Significance: minor
+Type: add
+
+Added documentation for payment methods API endpoint

docs/rest-api/source/includes/wp-api-v3/customer.md

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+# Customer
+
+The Customers API endpoints provide access to customer data. This includes payment methods and other key information useful for your application.
+
+
+## Get customer's payment methods
+
+_@since v6.6.0_
+
+Return all customer's payment methods.
+
+### Error codes
+
+-   `rest_forbidden` - indicates that the user or application does not have the necessary permissions to perform the requested action.
+
+### HTTP request
+
+<div class="api-endpoint">
+  <div class="endpoint-data">
+    <i class="label label-get">GET</i>
+    <h6>/wp-json/wc/v3/payments/customers/&lt;customer_id&gt;/payment_methods</h6>
+  </div>
+</div>
+
+```shell
+curl -X GET https://example.com/wp-json/wc/v3/payments/customers/cus_123456/payment_methods \
+  -u consumer_key:consumer_secret \
+  -H "Content-Type: application/json"
+```
+
+> JSON response example:
+
+```json
+[
+  {
+    "id": "pm_1AxXc2a5dGhIZQYPlaLbKj1Z",
+    "type": "card",
+    "billing_details": {
+      "address": {
+        "city": "Los Angeles",
+        "country": "US",
+        "line1": "123 Anywhere St",
+        "line2": null,
+        "postal_code": "90002",
+        "state": "CA"
+      },
+      "email": "[email protected]",
+      "name": "John Doe",
+      "phone": null
+    },
+    "card": {
+      "brand": "visa",
+      "last4": "1122",
+      "exp_month": 10,
+      "exp_year": 2028
+    }
+  },
+  {
+    "id": "pm_2BcYd3e6hKjZLQWQmMnOjK45",
+    "type": "card",
+    "billing_details": {
+      "address": {
+        "city": "New York",
+        "country": "US",
+        "line1": "456 Broadway Ave",
+        "line2": null,
+        "postal_code": "10012",
+        "state": "NY"
+      },
+      "email": "[email protected]",
+      "name": "Jane Smith",
+      "phone": null
+    },
+    "card": {
+      "brand": "mastercard",
+      "last4": "3344",
+      "exp_month": 12,
+      "exp_year": 2027
+    }
+  }
+]
+
+```
+
+```json
+{
+  "code":"rest_forbidden",
+  "message":"Sorry, you are not allowed to do that.",
+  "data":{
+    "status":401
+  }
+}
+```