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

tool/output/README.md

@@ -0,0 +1,23 @@
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/stl-container-output/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ STL 容器快捷输出
+
+对吴老师的代码进行了修改，[点击此处直通代码](https://github.com/Light-City/CPlusPlusThings/tree/master/tool/output/output_container.h)
+
+输入：
+
+```cpp
+map<int, int> mp{
+            {1, 1},
+            {2, 4},
+            {3, 9}};
+    cout << mp << endl;
+```
+
+输出结果显示：
+
+```cpp
+{ 1 => 1, 2 => 4, 3 => 9 }
+```
+
+####