Add "url" key in dbal configuration

"url" key exist in default doctrine configuration created from flex (doctrine.yaml)

Author: yordandv

reference/configuration/doctrine.rst

@@ -293,6 +293,7 @@ The following block shows all possible configuration keys:
                 user:                 user
                 password:             secret
                 driver:               pdo_mysql
+                url:                  mysql://db_user:db_password@127.0.0.1:3306/db_name 
                 # the DBAL driverClass option
                 driver_class:         MyNamespace\MyDriverImpl
                 # the DBAL driverOptions option