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/decltype/README.md

@@ -1,10 +1,12 @@
-# decltype那些事
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/decltype/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ decltype 类型推导
 
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 ## 1.基本使用
 decltype的语法是:
@@ -69,7 +71,7 @@ auto multiply(T x, T y)->decltype(x*y)
 }
 ```
 
-完整代码见：[decltype.cpp](decltype.cpp)
+完整代码见：[decltype.cpp](https://github.com/Light-City/CPlusPlusThings/tree/master/basic_content/decltype/decltype.cpp)
 
 ## 2.判别规则
 
@@ -135,4 +137,4 @@ decltype(Func(1)) var13=true;//const bool
 decltype(i++) var14 = i;//int i++返回右值
 ```
 
-学习参考：https://www.cnblogs.com/QG-whz/p/4952980.html
+学习参考：https://www.cnblogs.com/QG-whz/p/4952980.html