Add the asciidoctor-iconify extension

[mmattel]

https://github.com/lask79/asciidoctor-iconify

asciidoctor-iconify is an extension for asciidoctor.js and Antora provides an inline macro that inserts an icon into the document based on Iconify. The icon is rendered as an inline image. It uses the the iconify web component where javascript is used to dynamically replace the icon with an SVG image.

By using this macro you get access to > 200.000 icons from > 100 icon collections.

Note, needs some addon in docs-ui.