Improve documentation for installation

docs/source/development/code-quality.rst

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+Code style, code quality and other tools
+########################################
+
+PHP-cs-fixer
+============
+
+For development, you will also have to install:
+
+- `php-cs-fixer <https://cs.symfony.com/>`_
+
+We also encourage you to use tools like `phpstan <https://phpstan.org>`_ and `rector <https://getrector.com>`_.
+
+For running php-cs-fixer:
+
+.. code-block:: bash
+
+   symfony composer php-cs-fixer
+
+Execute tests
+=============
+
+.. code-block:: bash
+
+   symfony composer exec phpunit -- /path/to_your_test.php
+
+Note that IDE like PhpStorm should be able to run tests, even KernelTestcase or WebTestCase, `from within their interfaces <https://www.jetbrains.com/help/phpstorm/using-phpunit-framework.html#run_phpunit_tests>`_.
+
+Execute rector
+==============
+
+.. code-block:: bash
+
+   symfony composer exec rector -- process
+