Texinfo version 6.6dev has support for multi-level indices which we were supporting in a hackish way (based on the original LaTeX version). Since 6.6dev has it, let's use it.
But this means when we run CI, we have to clone our own copy to get it since 6.6dev isn't available as a package yet.
This works to build the cmu-user doc. Need to check the other texi docs, but cmu-user works, so getting the rest updated should be easy.