[...] In Sphinx, this is just a matter of reading the LilyPond input
in a Python extension and inserting the output at that point, as is
already done in
https://pypi.org/project/sphinxcontrib-lilypond/
or the alternative
https://github.com/sphinx-notes/lilypond
Which means that there is no longer a need to parse and pre-process
the source ourselves with regexes in lilypond-book, with the
associated build system fuss.
Note that it's actually a nice thing that LilyPond uses its own tool,
`lilypond-book`, for building its own documentation. This is good for
testing...