minor #9322 Update dbal.rst (stormoPL)

This PR was squashed before being merged into the 4.0 branch (closes #9322).

Discussion
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Update dbal.rst

Current documentation steps do not work for fresh Symfony 4 instance. Doctrine.yml file does not exist and DBAL installation from Composer does not get recognized or autowired. DoctrineBundle is a way to use both Doctrine ORM on the top of Doctrine DBAL or just pure DBAL.

Commits
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60d14ab Update dbal.rst

Author: javiereguiluz

doctrine/dbal.rst

@@ -21,7 +21,13 @@ makes it easy to execute queries and perform other database actions.
     Read the official Doctrine `DBAL Documentation`_ to learn all the details
     and capabilities of Doctrine's DBAL library.
 
-To get started, configure the ``DATABASE_URL`` environment variable in ``.env``:
+First, install the Doctrine bundle:
+
+.. code-block:: terminal
+
+    composer require doctrine/doctrine-bundle
+
+Then configure the ``DATABASE_URL`` environment variable in ``.env``:
 
 .. code-block:: text