Docs: reorganize documentation directory structure

Docs: reorganize documentation directory structure

* rename manuals by stripping "lilypond-" prefix from all source file names
  and HTML and PDF output (it is kept in Info output, where it is necessary),

* move manuals in Documentation/user one directory higher, in addition
  rename lilypond-program to application, and put .itely files in
  subdirectories,

* move Documentation/topdocs/NEWS.tely to Documentation,

* move Documentation/devel/contrib-guide.texi to
  Documentation/contributor.texi,

* move input/lsr/lilypond-snippets.tely to Documentation/, move the
  rest input/lsr to Documentation/snippets,

* move input/new to Documentation/snippets/new,

* move input/texidocs contents to
  Documentation/<language_codes>/texidocs, splitting texidocs by
  language,

* do the same changes in all translations when applicable,

* move Documentation/devel/index.html.in back to Documentation/deve.html.in.

'make all', 'make doc' and 'make dist' have been tested, although tons
of broken HTML and PDF links and Info cross-references are expected.

Maintenance makefiles (Documentation/GNUmakefile) and scripts
(scripts/auxiliar) are known to be broken, they will be updated in a
future commit.

TODO: Documentation/GNUmakefile is still a bit of a mess.