textproc/rubygem-asciidoctor-diagram: create port

Asciidoctor Diagram is a set of Asciidoctor extensions that enable you to add
diagrams, which you describe using plain text, to your AsciiDoc document.

WWW: https://rubygems.org/gems/asciidoctor-diagram/

PR: 232056
Submitted by: Šimun Mikecin <numisemis@yahoo.com>