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/vptr_vtable/README.md

@@ -1,10 +1,12 @@
-# 深入浅出C++虚函数的vptr与vtable
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-things/vptr-vtable/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ 虚指针与虚表机制
 
 ## 关于作者：
 
 个人公众号：
 
-![](../img/wechat.jpg)
+![](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/img/wechat.jpg)
 
 
 
@@ -24,7 +26,7 @@
 
 开始看代码之前，为了方便大家理解，这里给出调用图：
 
-![base](./img/base.jpg)
+![base](https://raw.githubusercontent.com/Light-City/CPlusPlusThings/master/basic_content/vptr_vtable/img/base.jpg)
 
 代码全部遵循标准的注释风格，相信大家看了就会明白，不明白的话，可以留言！
 
@@ -215,5 +217,4 @@ Dump of assembler code for function Derived::fun2():
    0x0000000000400efd <+41>:	leaveq 
    0x0000000000400efe <+42>:	retq   
 End of assembler dump.
-```
-
+```