Better callout rendering

Here is how callouts get rendered in Boost.Capy, which uses the look from this repo:
https://develop.capy.cpp.al/capy/2.cpp20-coroutines/2c.machinery.html#_the_compiler_transformation

The callout names in the source code render as text ("open paren" followed by number followed by "close paren"). This makes it look like a syntax error in the source code. Other Antora generated pages, render nice circles, like this one: https://docs.asciidoctor.org/asciidoc/latest/verbatim/callouts/

Can this be changed in this repo also?

Compare the callouts in the linked Boost.Capy page with the circular callouts shown in the Asciidoctor documentation. Then locate this repository’s callout-rendering configuration or templates and identify where the source-text labels are produced. Done means generated documentation displays callout names as circles rather than parenthesized text.

Written by the indexing model from the issue text.