doc: update documentation

- use namespace bracket all content in documentation to reduce useless namespace prefix.
- change the argument type of AbstractSetting and CoreManager to yycc_u8string_view instead of const yycc_char8_t*.
- throw exception if given setting name is invalid in ConfigManager, instead of slient fallback.

doc/src/io_helper.dox

@@ -1,3 +1,4 @@
+namespace YYCC::IOHelper {
 /**
 
 \page io_helper IO Helper
@@ -40,4 +41,5 @@ There is a simple example:
 FILE* fs = YYCC::IOHelper::FOpen(YYCC_U8("/path/to/file"), YYCC_U8("rb"));
 \endcode
 
-*/
+*/
+}