Documented the security:check command

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #4051
Advising to run the command on a regular basis imho calls for integration into build environments. :) We can simply add a tip after this paragraph like this:

```rst
.. tip::

    The ``security:check`` command terminates with a non-zero exit code if
    any of your dependencies is affected by a known security vulnerability.
    Therefore, you can easily integrate it in your build process.
```