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/volatile/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/volatile/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ volatile 关键字
+
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 ## 1.volatile
 
@@ -117,6 +121,5 @@ return a * a;
 
 代码学习：
 
-- [noopt_vola.cpp](./noopt_vola.cpp)
-- [volatile.cpp](./volatile.cpp)
-
+- [noopt_vola.cpp](https://github.com/Light-City/CPlusPlusThings/tree/master/basic_content/volatile/noopt_vola.cpp)
+- [volatile.cpp](https://github.com/Light-City/CPlusPlusThings/tree/master/basic_content/volatile/volatile.cpp)