[LyX/master] LyXHTML: switch the doctype to (X)HTML5 and only output

[Pavel Sanda]

...
CXX      insets/InsetSpecialChar.o
insets/InsetSpecialChar.cpp: In function 'std::string lyx::{anonymous}::specialCharKindToXMLEntity(lyx::InsetSpecialChar::Kind)':
insets/InsetSpecialChar.cpp:569:1: warning: control reaches end of non-void function [-Wreturn-type]
  569 | }
      | ^
...