Fix use of relative in docs and restore UTF-8 strings

Fix use of relative in docs and restore UTF-8 strings

* replace bare \relative with "\relative c'" -- I didn't check
systematically, there must remain some occurences of bare
\relative.

* fix misuse of relative lilypond-book fragment option -- cases of
misuse were probably not fixed because relative without fragment used
to have no effect on ly snippets.

* also restore mangled UTF-8 strings and add misssing accent in "Chéri".