[RFC] Documentation: Split all manuals at nodes
extract_texi_filenames.py, responsible for producing the .xref-map
files, used to implement a 'custom' split mode. This placed @unnumbered*
sections into the same file, deviating from regular info output. It
also required quite a bit of code both in the script and our texi2html
init file. Over time, this setup gained a number of special cases to
deal with LilyPond's manual structure making it really complex. For
example, top-level @unnumbereds were given their own file in most
manuals, except in the Changes document which used to be one page only.
As an example for the changes caused by this, the first difference
in the LM is "2.2.2 Simple notation" where "Pitches", "Durations",
etc., used to be put on one page. After removing this 'custom' split
mode, these @unnumberedsubsubsec are put into their own files because
they have a node associated with them. Similar results can be observed
in all manuals, except the website which was always split at nodes.
Then add @menus to the Changes document.