"Avoiding (((index))) in header lines" breaks formatting of lists

[hansu]

#3671 breaks the formatting of lists which follows directly a heading.

For example:

https://linuxcnc.org/docs/devel/html/getting-started/about-linuxcnc.html
https://www.linuxcnc.org/docs/devel/html/config/core-components.html#sec:spindle-pins
https://linuxcnc.org/docs/devel/html/gcode/m-code.html#mcode:m2-m30

and several more.

I wonder if we can just remove those indices? According to the docs it is used for generating DocBook index entries: https://asciidoc-py.github.io/userguide.html#_indexes
But I tried it with remove one and the index in the PDF version was fine. So any objections?

@smoe please check the next time(s) that your po4a optimized changes don't break the formatting. This one was even visible with VSCode's internal AsciiDoctor preview.
It takes a lot of time to back trace those things which could be spent on other issues ...