MusicXML: Insert the section headers as pure texinfo code

MusicXML: Insert the section headers as pure texinfo code

Unfortunately, we can't yet get rid of the AAA-intro-regression.ly
hack, because the description in that file tries to include the
lilypond version. lilypond-book now supports @liypondversion, but
unfortunately, that's not available during docs building,
so I have to leave that one .ly in there :((((