book: motivate §7.1 with why standardized threading matters

§7.1 dove straight into the std::thread API; add a lead explaining that
pre-C++11 there was no standard threading (only platform pthread/Windows
APIs), so C++11's thread library is what makes portable concurrency
possible.

Author: changkun

book/en-us/07-thread.md

@@ -12,6 +12,8 @@ order: 7
 
 *(since C++11)*
 
+Before C++11, the C++ standard library had no notion of threads at all: writing concurrent code meant reaching for platform-specific APIs such as POSIX `pthread` or the Windows thread API, which made that code hard to port. C++11 brought threading into the language standard for the first time, so the same concurrent code now builds across platforms.
+
 `std::thread` is used to create an execution thread instance, so it is the basis for all concurrent programming. It needs to include the `<thread>` header file when using it.
 It provides a number of basic thread operations, such as `get_id()` to get the thread ID of the thread being created, use `join()` to join a thread, etc., for example:
 

book/zh-cn/07-thread.md

@@ -12,6 +12,8 @@ order: 7
 
 *（C++11）*
 
+在 C++11 之前，C++ 标准库根本没有线程的概念：要编写并发代码，只能依赖平台特定的 API（如 POSIX 的 `pthread` 或 Windows 线程 API），这让代码难以跨平台移植。C++11 首次把线程纳入语言标准，使同一份并发代码得以跨平台构建。
+
 `std::thread` 用于创建一个执行的线程实例，所以它是一切并发编程的基础，使用时需要包含 `<thread>` 头文件，
 它提供了很多基本的线程操作，例如 `get_id()` 来获取所创建线程的线程 ID，使用 `join()` 来等待一个线程结束（与该线程汇合）等等，例如：