lilypond-book: Drop support for 'addversion'
This option was only used within the Lilypond documentation and relied on
the @version
macro, autogenerated by scripts/build/create-version-itexi.py.
Rationale for dropping this functionality:
- There were only three occurrences of 'addversion' left in the current docs, for none of them was clear which difference could justify the need for adding a version number, while the majority of doc snippets does not have any
- it needed a complicated nesting of Texinfo environments like
@format
@exampleindent 0
@example
\version ...
to give consistent output through all output formats (PDF, Info, HTML).
For the preceding discussion, see https://lists.gnu.org/archive/html/lilypond-devel/2020-07/msg00006.html