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

design_pattern/singleton/README.md

@@ -1,4 +1,6 @@
-## C++设计模式之单例模式
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/design-patterns/singleton-pattern/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ 单例模式
 
 ## 0.导语
 
@@ -325,4 +327,4 @@ public:
 
 > https://www.cnblogs.com/liyuan989/p/4264889.html
 
-> https://segmentfault.com/a/1190000015950693
+> https://segmentfault.com/a/1190000015950693