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/dbg-macro/README.md

@@ -0,0 +1,70 @@
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/dbg-macro/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ dbg-macro 调试宏
+
+打日志是 C++ 开发中必不可少的一种 debug 方式，dbg-macro 受 Rust 语言中 的 `dbg` 启发，提供比 `printf` 和 `std::cout` 更好的宏函数。主要有如下特点：
+
+- 美观的彩色输出（当输出不是交互式终端时，颜色将自动禁用）
+- 兼容 C++11，并且是 header-only
+- 支持基础类型和 STL 容器类型的输出
+- 可以输出文件名、行号、函数名和原始表达式
+
+例如下面的代码：
+
+```cpp
+#include <vector>
+#include <dbg.h>
+
+// You can use "dbg(..)" in expressions:
+int factorial(int n) {
+  if (dbg(n <= 1)) {
+    return dbg(1);
+  } else {
+    return dbg(n * factorial(n - 1));
+  }
+}
+
+int main() {
+  std::string message = "hello";
+  dbg(message);  // [example.cpp:15 (main)] message = "hello" (std::string)
+
+  const int a = 2;
+  const int b = dbg(3 * a) + 1;  // [example.cpp:18 (main)] 3 * a = 6 (int)
+
+  std::vector<int> numbers{b, 13, 42};
+  dbg(numbers);  // [example.cpp:21 (main)] numbers = {7, 13, 42} (size: 3) (std::vector<int>)
+
+  dbg("this line is executed");  // [example.cpp:23 (main)] this line is executed
+
+  factorial(4);
+
+  return 0;
+}
+```
+
+效果如下：
+
+![](https://camo.githubusercontent.com/3cde47c8db560b3ed42763f2ef306d78d4b19087/68747470733a2f2f692e696d6775722e636f6d2f4e4845596b39412e706e67)
+
+## 安装
+
+```bash
+git clone https://github.com/sharkdp/dbg-macro
+sudo ln -s $(readlink -f dbg-macro/dbg.h) /usr/include/dbg.h
+```
+
+这样，你就可以在任何地方使用 `dbg.h` 了。
+
+### 在 Arch Linux 上
+
+```bash
+yay -S dbg-macro
+```
+
+### 使用 vcpkg
+
+```bash
+vcpkg install dbg-macro
+```
+
+更多内容详见 [官方文档](https://github.com/sharkdp/dbg-macro)。

tool/like_python/README.md

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+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/cpp-jupyter-notebook/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ Jupyter Notebook 交互
+
+在Python中我们可以使用`Jupyter Notebook`直接看到结果，例如：
+
+```c
+l = [1,2]
+l
+```
+
+直接输出：
+
+```
+[1,2]
+```
+
+那当使用C++的时候，例如：
+
+```cpp
+map<string, int> mp{
+    {"one",   1},
+    {"two",   2},
+    {"three", 3},
+    {"four",  4}
+};
+```
+
+如果要输出，就得循环遍历，可否直接输出结果呢？
+
+so easy!!! `Jupyter Notebook`可以解决一切问题，哈哈～
+
+## 如何在Jupyter中玩C++？
+
+在github上有一个仓库，如下所示：
+
+> https://github.com/QuantStack/xeus-cling
+
+`xeus-cling` 是一个用于C++的Jupyter内核，基于C++解释器和Jupyter协议xeus的原生实现。
+
+目前，支持Mac与Linux，但不支持Windows。
+
+安装也是非常简单，首先安装好Anaconda，在里面创建一个虚拟环境：
+
+```
+conda create -n cling
+```
+
+切换进去：
+
+```
+conda activate cling
+```
+
+给新环境安装`jupyter`和`notebook`
+
+```
+conda install jupyter notebook
+```
+
+使用`conda-forge`安装`xeus-cling`
+
+```
+conda install xeus-cling -c conda-forge
+```
+
+为了加速安装，请记得给Anaconda配置源！
+
+检查是否安装好了内核(kernel)：
+
+```
+jupyter kernelspec list
+```
+
+输出：
+
+```cpp
+python3    /home/xxx/anaconda3/envs/cling/share/jupyter/kernels/python3
+xcpp11     /home/xxx/anaconda3/envs/cling/share/jupyter/kernels/xcpp11
+xcpp14     /home/xxx/anaconda3/envs/cling/share/jupyter/kernels/xcpp14
+xcpp17     /home/xxx/anaconda3/envs/cling/share/jupyter/kernels/xcpp17
+```
+
+打开`Jupyter Notebook`，就可以看到看到kernel了。
+
+启动`Jupyter Notebook`：
+
+```
+jupyter-notebook
+```
+
+## 如何在Jupyter中玩C？
+
+只需要安装c kernel即可！
+
+可以直接在当前环境中创建c kernel，也可以新开一个环境安装，下面是在当前环境中直接安装。
+
+```
+pip install jupyter-c-kernel
+install_c_kernel
+jupyter kernelspec list
+```
+
+此时，就输出：
+
+```cpp
+c          /home/light/anaconda3/envs/cling/share/jupyter/kernels/c
+python3    /home/light/anaconda3/envs/cling/share/jupyter/kernels/python3
+xcpp11     /home/light/anaconda3/envs/cling/share/jupyter/kernels/xcpp11
+xcpp14     /home/light/anaconda3/envs/cling/share/jupyter/kernels/xcpp14
+xcpp17     /home/light/anaconda3/envs/cling/share/jupyter/kernels/xcpp17
+```
+
+启动`Jupyter Notebook`：
+
+```
+jupyter-notebook
+```

tool/look/README.md

@@ -0,0 +1,5 @@
+> 本文档由 [C++那些事网站](https://light-city.github.io/stories_things/cpp-tools/cpp-insights/) 同步生成，网站版本含多语言与更新排版。
+
+# C++ Insights 编译期观察
+
+- [C++ Insights — 查看编译器眼中的代码](https://cppinsights.io/)

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 }
+```
+
+####