docs: address community issue feedback (#5 #338 #142 #246 #136 #303)

- const README: rewrite the space-saving claim per issue #5 follow-ups
  (constant-expression const vs #define immediates; which const qualify
  as constant expressions)
- const README: fix the class-section diagnosis per issue #338 (overload
  resolution picks the const overload; the restriction only surfaces
  once that overload is removed)
- sizeof README: virtual-inheritance result is compiler-dependent
  (GCC merges vbptr+vfptr -> 16, MSVC keeps them separate -> 32) #246
- static README: new section on internal linkage / access restriction
  with anonymous-namespace comparison #142
- 10-day practice: full problem list table + fixed day links #136
- new design_pattern/singleton/singleton_default.hpp: reusable
  thread-safe CRTP singleton template, compile+concurrency tested #303

basic_content/explicit/README.md

@@ -1,16 +1,18 @@
-# explicit(显式)关键字那些事
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/explicit/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ explicit 显式转换
 
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 - explicit 修饰构造函数时，可以防止隐式转换和复制初始化
 - explicit 修饰转换函数时，可以防止隐式转换，但按语境转换除外
 
 
-代码参见:[.explicit.cpp](./explicit.cpp)
+代码参见:[.explicit.cpp](https://github.com/Light-City/CPlusPlusThings/tree/master/basic_content/explicit/explicit.cpp)
 
 参考链接：
-> https://stackoverflow.com/questions/4600295/what-is-the-meaning-of-operator-bool-const
+> https://stackoverflow.com/questions/4600295/what-is-the-meaning-of-operator-bool-const