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/union/README.md

@@ -1,10 +1,12 @@
-# UNION那些事
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/union/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ union 联合体
 
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 联合（union）是一种节省空间的特殊的类，一个 union 可以有多个数据成员，但是在任意时刻只有一个数据成员可以有值。当某个成员被赋值后其他成员变为未定义状态。联合有如下特点：
 
@@ -15,4 +17,4 @@
 - 不能含有虚函数
 - 匿名 union 在定义所在作用域可直接访问 union 成员
 - 匿名 union 不能包含 protected 成员或 private 成员
-- 全局匿名联合必须是静态（static）的
+- 全局匿名联合必须是静态（static）的