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/bit/README.md

@@ -1,8 +1,12 @@
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/bit/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ 位域
+
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 ## Bit field 是什么？
 
@@ -108,7 +112,7 @@ struct stuff
 
 这里 stuff.field1 与 stuff.field2 之间有一个 2 Bits 的空隙，stuff.field3 则存储在下一个 unsigned int 中，该结构现在大小为 3 * 32 = 96 Bits。
 
-学习代码见：[bit.cpp](bit.cpp)
+学习代码见：[bit.cpp](https://github.com/Light-City/CPlusPlusThings/tree/master/basic_content/bit/bit.cpp)
 
 ## 位域的初始化和位的重映射