Split user manual by creating "program-usage" manual

Split user manual by creating "program-usage" manual

The new manual is located in Documentation/user/lilypond-program.tely;
it includes install.itexi (moved from topdocs/INSTALL.texi),
running.itely, lilypond-book.itely and converters.itely.

As a consequence of this change, this huge commit also includes
- modified makefiles and add_html_footer
- updated translation infrastructure

- new @rprogram and @ruser Texinfo macros for cross-references between
the two new manuals
- fixed cross-references between the two manuals

- updated documentation index
- modified PDF front page (big subtitle added)
- lilypond.tely cleanup

- translated docs: PO update, "program usage" translated only in
French, lilypond.tely cleanup for all languages