minor #11332 Add doctrine orm installation instructions (LavaSlider, Toflar)

This PR was merged into the 3.4 branch.

Discussion
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Add doctrine orm installation instructions

Follow up branch of #10798 including the requested changes. Also changed the base branch to 3.4 because it applies there too :)

Commits
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6d164c3 Addressed issues raised in (#10798)
d579eea Add Doctrine ORM installation instructions.

Author: wouterj

best_practices/business-logic.rst

@@ -191,6 +191,15 @@ library or strategy you want for this.
 
 In practice, many Symfony applications rely on the independent
 `Doctrine project`_ to define their model using entities and repositories.
+
+Doctrine support is not enabled by default in Symfony. So to use Doctrine
+as shown in the examples below you will need to install :doc:`Doctrine ORM support </doctrine>`
+by executing the following command:
+
+.. code-block:: terminal
+
+    $ composer require symfony/orm-pack
+
 Just like with business logic, we recommend storing Doctrine entities in the
 AppBundle.
 
@@ -301,7 +310,7 @@ the following command to install the Doctrine fixtures bundle:
 
 .. code-block:: terminal
 
-    $ composer require "doctrine/doctrine-fixtures-bundle"
+    $ composer require doctrine/doctrine-fixtures-bundle
 
 Then, enable the bundle in ``AppKernel.php``, but only for the ``dev`` and
 ``test`` environments::
@@ -357,6 +366,7 @@ Next: :doc:`/best_practices/controllers`
 
 .. _`full definition`: https://en.wikipedia.org/wiki/Business_logic
 .. _`Doctrine project`: http://www.doctrine-project.org/
+.. _`Doctrine ORM support`: https://symfony.com/doc/current/doctrine.html
 .. _`fixture class`: https://symfony.com/doc/current/bundles/DoctrineFixturesBundle/index.html#writing-simple-fixtures
 .. _`PSR-1`: https://www.php-fig.org/psr/psr-1/
 .. _`PSR-2`: https://www.php-fig.org/psr/psr-2/