Doc: IR: Rewrite and improve document-music.scm

document-music.scm generates the "Music definitions" section of the
Internals reference, with the subsections "Music expressions", "Music
classes" and "Music properties".

Rewrite this code within the new framework developed around
document-translation.scm. In the process, improve music class
documentation to better reflect the class hierarchy and provide
backlinks from music properties.

Clean up now-obsolete definitions from documentation.lib.

As before with output objects and output object properties, the
current file structure necessitates to put various related definitions
into document-translation.scm rather than document-music.scm because
they need to be available for cross-referencing in the translator
documentation. A reorganisation of that structure seems in order.
document-backend.scm: Small simplification and correction

* Use hash-map->list instead of a more complicated hash-fold call

* in assignments, actually use the sorted property list created there