minor #17011 [Security] Add link to page with expressions (HypeMC)

This PR was merged into the 4.4 branch.

Discussion
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[Security] Add link to page with expressions

The [page that explains how expressions can be used for access control](https://symfony.com/doc/4.4/security/expressions.html) isn't linked anywhere in the security section, making it really hard to find. The only place where it is linked is on the [expression syntax page](https://symfony.com/doc/4.4/components/expression_language/syntax.html).

Commits
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b805007 [Security] Add link to page with expressions

Author: javiereguiluz

security.rst

@@ -1092,6 +1092,7 @@ Authorization (Denying Access)
 .. toctree::
     :maxdepth: 1
 
+    security/expressions
     security/voters
     security/securing_services
     security/access_control

security/expressions.rst

@@ -1,7 +1,7 @@
 .. index::
    single: Expressions in the Framework
 
-Security: Complex Access Controls with Expressions
+Complex Access Controls with Expressions
 ==================================================
 
 .. seealso::
@@ -14,7 +14,7 @@ accepts an :class:`Symfony\\Component\\ExpressionLanguage\\Expression` object::
 
     // src/Controller/MyController.php
     namespace App\Controller;
-    
+
     use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
     use Symfony\Component\ExpressionLanguage\Expression;
     use Symfony\Component\HttpFoundation\Response;