minor #6585 6555 link to Download instructions page & Windows executable (snoek09)

This PR was squashed before being merged into the 2.3 branch (closes #6585).

Discussion
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6555 link to Download instructions page & Windows executable

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | #6555

I decided to not copy any instructions as mentioned on https://getcomposer.org/download/

> WARNING: Please do not redistribute the install code. It will change with every version of the installer. Instead, please link to this page.

Commits
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1967b9d 6555 link to Download instructions page & Windows executable

Author: xabbuh

cookbook/composer.rst

@@ -14,35 +14,24 @@ following sections.
 Install Composer on Linux and Mac OS X
 --------------------------------------
 
-To install Composer on Linux or Mac OS X, execute the following two commands:
+#. Run the installer as described in `the official Composer documentation`_;
+#. Execute the following command to move the ``composer.phar`` to a directory that is in your path:
 
-.. code-block:: bash
+   .. code-block:: bash
 
-    $ curl -sS https://getcomposer.org/installer | php
-    $ sudo mv composer.phar /usr/local/bin/composer
-
-.. note::
-
-    If you don't have ``curl`` installed, you can also just download the
-    ``installer`` file manually at https://getcomposer.org/installer and
-    then run:
-
-    .. code-block:: bash
-
-        $ php installer
-        $ sudo mv composer.phar /usr/local/bin/composer
+       $ sudo mv composer.phar /usr/local/bin/composer
 
 Install Composer on Windows
 ---------------------------
 
-Download the installer from `getcomposer.org/download`_, execute it and follow
-the instructions.
+Download the installer from `getcomposer.org`_, execute it and follow the instructions.
 
 Learn more
 ----------
 
 Read the `Composer documentation`_ to learn more about its usage and features.
 
 .. _`Composer`: https://getcomposer.org/
-.. _`getcomposer.org/download`: https://getcomposer.org/download
 .. _`Composer documentation`: https://getcomposer.org/doc/00-intro.md
+.. _`getcomposer.org`: https://getcomposer.org/Composer-Setup.exe
+.. _the official Composer documentation: https://getcomposer.org/download