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/library_encoding.dox

@@ -1,3 +1,4 @@
+namespace YYCC {
 /**
 
 \page library_encoding Library Encoding
@@ -224,3 +225,4 @@ Linux user do not need care this.
 Because almost Linux distro use UTF8 in default.
 
 */
+}