[Cookbook][Security] Added doc for x509 pre authenticated listener

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | 2.3+
| Fixed tickets | -

This can be merged in 2.3. Regards.
I'd reword this also:

> Enable the x509 authentication for a particular firewall in the security configuration: