Add a note to comment default virtual host for nginx configuration on main how-to page and in documentation

Hi,
on our main zabbix.com page in Download section we have simple quick how-to to get Zabbix server running

When you choose nginx as web server it by default is listening default port without zabbix URI that conflicts with default nginx web page from nginx.conf.
Let's put a note to comment out server section in /etc/nginx/nginx.conf.

Regards, Elina