Clickable icons for custom anchors

[eric-schneider]

We have a legacy ticket open to enhance our old Antora UI to provide clickable icons for custom anchors (DOC-4076). This was a feature that existed in the old DITA UI but did not get ported into our Antora UI.

The request is to provide clickable anchor link icons, like the auto-generated ones we currently have on section titles, but on elements where a custom AsciiDoc ID has been defined. These can be block and inline elements, such as list items, tables, etc.