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/pointer_refer/README.md

@@ -1,10 +1,12 @@
-# 引用与指针那些事
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/pointer-vs-reference/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ 指针与引用
 
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 ## 1.引用与指针
 
@@ -171,4 +173,4 @@ End of assembler dump.
 
 C++中引入了引用操作，在对引用的使用加了更多限制条件的情况下，保证了引用使用的安全性和便捷性，还可以保持代码的优雅性。在适合的情况使用适合的操作，引用的使用可以一定程度避免“指针满天飞”的情况，对于提升程序稳定性也有一定的积极意义。最后，指针与引用底层实现都是一样的，不用担心两者的性能差距。
 
-上述部分参考自：<http://irootlee.com/juicer_pointer_reference/#>
+上述部分参考自：<http://irootlee.com/juicer_pointer_reference/#>