Draft: Add Nginx configuration example

Add nginx_example.conf.

Copied from https://gitlab.com/vazhnov/smokeping_nginx/-/blob/main/best.conf

Because there is no possibility to create an issue in the repository https://salsa.debian.org/debian/smokeping, I need some clarification about next steps:

• better path,
• update deb configs,
• update documentation with https://gitlab.com/vazhnov/smokeping_nginx/-/blob/main/README.md

Also, because I'm the author of https://gitlab.com/vazhnov/smokeping_nginx, I can change the license to any other opensource you wish, for better compatibility.