minor #8855 Simplify the setup article (javiereguiluz)

This PR was squashed before being merged into the 4.0 branch (closes #8855).

Discussion
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Simplify the setup article

The removed contents were just a repetition of what you can read at https://symfony.com/doc/current/reference/requirements.html

Commits
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c19c90e Simplify the setup article

Author: javiereguiluz

setup.rst

@@ -49,8 +49,8 @@ First, move into your new project and install the server:
 
 .. code-block:: terminal
 
-    cd my-project
-    composer require server --dev
+    $ cd my-project
+    $ composer require server --dev
 
 To start the server, run:
 
@@ -62,6 +62,12 @@ Open your browser and navigate to ``http://localhost:8000/``. If everything is w
 you'll see a welcome page. Later, when you are finished working, stop the server
 by pressing ``Ctrl+C`` from your terminal.
 
+.. tip::
+
+    If you're having any problems running Symfony, your system may be missing
+    some technical requirements. Use the :doc:`Symfony Requirements Checker </reference/requirements>`
+    tool to make sure your system is set up.
+
 .. tip::
 
     If you're using a VM, you may need to tell the server to bind to all IP addresses:
@@ -89,35 +95,6 @@ Your project already has a sensible ``.gitignore`` file. And as you install more
 packages, a system called :ref:`Flex <flex-quick-intro>` will add more lines to that
 file when needed.
 
-Troubleshooting: The Requirements Checker
------------------------------------------
-
-If you're having any problems running Symfony, your system may be missing some
-`technical requirements`_.  Symfony has a "Requirements Checker" tool that you
-can use to easily make sure your system is set up. First, move into your project
-directory and install it:
-
-.. code-block:: terminal
-
-    $ composer require req-checker
-
-The ``req-checker`` utility adds two PHP scripts to your application:
-``vendor/bin/requirements-checker`` and ``public/check.php``. Run the first one
-from your terminal:
-
-.. code-block:: terminal
-
-    php vendor/bin/requirements-checker
-
-This will check your CLI environment. Run the second one from a browser (e.g.
-``http://localhost:8000/check.php``) to check your web server environment.
-
-Once you've fixed any issues, uninstall the requirements checker:
-
-.. code-block:: terminal
-
-    $ composer remove req-checker
-
 .. _install-existing-app:
 
 Setting up an Existing Symfony Project