General:

Zabbix now provides an official package for raspbian. (Which is simply awesome...)

The Raspbian option is available on https://www.zabbix.com/download en is documented on that page. 

Issue: 

The official documentation doesn't mention raspbian at all.

Suggestion:

I suggest adding something to https://www.zabbix.com/documentation/4.0/manual/installation/install_from_packages/debian_ubuntu (perhaps renaming the page to include raspbian too)