Texinfo doesn't currently support subtopics in the concept index. We can, however, make it work by making
@cpsubindex(topic,subtopic) create an entry "topic!subtopic" in the
.cp file. We can process this file to convert the entry to a four argument version where the fourth arg is the subtopic.
Then building the pdf file will have subtopics. Hurray! However, it's a little buggy. I blame
texindex not doing quite the right thing all the time.
We also added makefiles to make building and installing the docs easier. The CI rules use the makefiles.