book: group the table of contents into three Parts

Organize the contents into Part I (Language Core, ch1-3), Part II (The
Standard Library, ch4-9), Part III (By Standard Version, ch10-12), plus
an Appendices group, each with a one-line intro. The theme renders a
flat sidebar (no Part concept), so the grouping lives in the canonical
toc.md contents page.

Author: changkun

book/en-us/toc.md

@@ -3,7 +3,11 @@
 ## Table of Contents
 
 - [**Preface**](./00-preface.md)
-- [**Chapter 01 Towards Modern C++**](./01-intro.md)
+### Part I — Language Core
+
+*Enhancements to the language itself: how you declare and deduce types, and what happens at runtime.*
+
+- [**Chapter 01: Towards Modern C++**](./01-intro.md)
     + 1.1 Deprecated Features
     + 1.2 Compatibility with C
     + Further Readings
@@ -51,6 +55,10 @@
         + rvalue reference and lvalue reference
         + Move semantics
         + Perfect forwarding
+### Part II — The Standard Library
+
+*Batteries included: containers, smart pointers, regular expressions, concurrency, and the filesystem.*
+
 - [**Chapter 04: Containers**](./04-containers.md)
     + 4.1 Linear containers
         + `std::array`
@@ -96,6 +104,10 @@
         + Raw String Literal
         + Custom String Literal
     + 9.4 Memory Alignment
+### Part III — By Standard Version
+
+*A tour of what each recent standard adds: C++20, C++23, and the forthcoming C++26.*
+
 - [**Chapter 10: C++20**](./10-cpp20.md)
     + 10.1 Concept
     + 10.2 Module
@@ -125,6 +137,8 @@
       - std::execution
       - Saturation arithmetic
       - Other library facilities
+### Appendices
+
 - [**Appendix 1: Further Study Materials**](./appendix1.md)
 - [**Appendix 2: Modern C++ Best Practices**](./appendix2.md)
 - [**Appendix 3: Modern C++ Feature Index**](./appendix3.md)

book/zh-cn/toc.md

@@ -3,6 +3,10 @@
 ## 目录
 
 - [**序言**](./00-preface.md)
+### 第一部分　语言核心
+
+*语言自身的强化：如何声明与推导类型，以及运行期的增强。*
+
 - [**第 1 章 迈向现代 C++**](./01-intro.md)
     + 1.1 被弃用的特性
     + 1.2 与 C 的兼容性
@@ -51,6 +55,10 @@
         + 右值引用和左值引用
         + 移动语义
         + 完美转发
+### 第二部分　标准库
+
+*开箱即用的设施：容器、智能指针、正则表达式、并发与文件系统。*
+
 - [**第 4 章 标准库: 容器**](./04-containers.md)
     + 4.1 线性容器
         + `std::array`
@@ -96,6 +104,10 @@
         + 原始字符串字面量
         + 自定义字面量
     + 9.4 内存对齐
+### 第三部分　按标准版本
+
+*概览每个较新标准带来的内容：C++20、C++23 与即将到来的 C++26。*
+
 - [**第 10 章 C++20**](./10-cpp20.md)
     + 10.1 概念与约束
     + 10.2 模块
@@ -125,6 +137,8 @@
       - std::execution
       - 饱和算术
       - 其他库设施
+### 附录
+
 - [**附录 1：进一步阅读的学习材料**](./appendix1.md)
 - [**附录 2：现代 C++ 的最佳实践**](./appendix2.md)
 - [**附录 3：现代 C++ 特性索引**](./appendix3.md)