Documented the security:check command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4051
I've created the following link. Please tell me if the format is correct:

```
.. tip::

    You can check your Symfony application for known security vulnerabilities
    using the ``security:check`` command. See :doc:`</book/security/checking-vulnerabilities>`
```