Update security.rst to include missing link

The documentation for "How to Build a JSON Authentication Endpoint" (https://symfony.com/doc/current/security/json_login_setup.html) is missing in the Security documentation (https://symfony.com/doc/current/security.html#learn-more). This is valuable knowledge that IMO should be represented in the general Security documentation.

Author: mehlichmeyer

security.rst

@@ -985,6 +985,7 @@ Authentication (Identifying/Logging in the User)
 .. toctree::
     :maxdepth: 1
 
+    security/json_login_setup
     security/form_login_setup
     security/guard_authentication
     security/auth_providers