Recognize and preserve index section when assembling document (root level 0)

Asciidoctor supports index terms using indexterm:[<primary>, <secondary>, <tertiary>] and indexterm2:[<primary>]. See https://docs.asciidoctor.org/asciidoc/latest/sections/user-index/#index-terms. Asciidoctor PDF supports generating an index section from these terms by adding an index section at the end of the document:

[index]
== Index

Unfortunately, it seems Antora Assembler breaks this by replacing [index] with [discrete] (see Zulip chat).

Please allow the index section to be added without the use of an extension. Even better: add some kind of index: true option in Antora PDF to autogenerate the index, without even needing to add the [index] == Index section to the last page in the assembled document.