minor #4649 Linked the PDO/DBAL Session article from the Doctrine category (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #4649).

Discussion
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Linked the PDO/DBAL Session article from the Doctrine category

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #4416

Commits
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cb4784a Linked the PDO/DBAL Session article from the Doctrine category

Author: wouterj

cookbook/map.rst.inc

@@ -68,6 +68,7 @@
   * :doc:`/cookbook/doctrine/mapping_model_classes`
   * :doc:`/cookbook/doctrine/registration_form`
   * :doc:`/cookbook/doctrine/console`
+  * (configuration) :doc:`/cookbook/configuration/pdo_session_storage`
 
 * :doc:`/cookbook/email/index`
 
@@ -164,6 +165,7 @@
   * :doc:`/cookbook/session/locale_sticky_session`
   * :doc:`/cookbook/session/sessions_directory`
   * :doc:`/cookbook/session/php_bridge`
+  * (configuration) :doc:`/cookbook/configuration/pdo_session_storage`
 
 * **symfony1**