minor #20201 [Serializer] Add the yaml and xml config folders (lkolndeep)

This PR was merged into the 5.4 branch.

Discussion
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[Serializer] Add the yaml and xml config folders

Add the yaml and xml configuration folders for the example.

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39878b8 Add the yaml and xml config folders

Author: javiereguiluz

serializer.rst

@@ -209,6 +209,7 @@ You can also specify the context on a per-property basis::
 
     .. code-block:: yaml
 
+        # config/serializer/custom_config.yaml
         App\Model\Person:
             attributes:
                 createdAt:
@@ -218,6 +219,7 @@ You can also specify the context on a per-property basis::
     .. code-block:: xml
 
         <?xml version="1.0" encoding="UTF-8" ?>
+        <!-- config/serializer/custom_config.xml -->
         <serializer xmlns="http://symfony.com/schema/dic/serializer-mapping"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://symfony.com/schema/dic/serializer-mapping