minor #11833 Provide a link to Symfony client download instructions (danabrey)

This PR was submitted for the 4.3 branch but it was merged into the 4.2 branch instead (closes #11833).

Discussion
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Provide a link to Symfony client download instructions

Make it clearer how to install the Symfony web server directly from the Installing & Setting up the Symfony Framework page.

Commits
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d208080 Provide a link to Symfony client download instructions

Author: javiereguiluz

setup.rst

@@ -153,6 +153,7 @@ Go Deeper with Setup
 .. _`Stellar Development with Symfony`: http://symfonycasts.com/screencast/symfony
 .. _`Composer`: https://getcomposer.org/
 .. _`installing Composer`: https://getcomposer.org/download/
+.. _`Download the Symfony client`: https://symfony.com/download
 .. _`the Security Checker`: https://github.com/sensiolabs/security-checker#integration
 .. _`The Symfony Demo application`: https://github.com/symfony/demo
 .. _`symfony/symfony-demo`: https://github.com/symfony/demo