Add Icinga Director to list of components capable of strict CSP

[martialblog]

Hi,

I updated the docs for the Content Security Policy (CSP). The Director was missing from the list.

Was implemented in https://github.com/Icinga/icingaweb2-module-director/pull/2857

Edit: I also added a hint about how to enable CSP in Icinga Web.

Cheers Markus