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/win_fct_helper.dox

@@ -1,3 +1,4 @@
+namespace YYCC::WinFctHelper {
 /**
 
 \page win_fct_helper Windows Function Helper
@@ -9,9 +10,10 @@ It will be entirely invisible in other platforms.
 
 Currently this namespace has following functions:
 
-\li YYCC::WinFctHelper::GetCurrentModule: Get the handle to current module.
-\li YYCC::WinFctHelper::GetTempDirectory: Get temporary directory in Windows.
-\li YYCC::WinFctHelper::GetModuleFileName: Get the path to module in file system by given handle.
-\li YYCC::WinFctHelper::GetLocalAppData: Get the path inside \%LOCALAPPDATA\%
+\li #GetCurrentModule: Get the handle to current module.
+\li #GetTempDirectory: Get temporary directory in Windows.
+\li #GetModuleFileName: Get the path to module in file system by given handle.
+\li #GetLocalAppData: Get the path inside \%LOCALAPPDATA\%
 
-*/
+*/
+}