docs: 修正 README 并改进工具链检测

- README: 修正路径引用 (Tools→Toolchain, 去掉不存在的 Projects/ 层级)
- README: 新增快速开始 / 工程结构 / 板级配置 / 编译烧录章节
- README: 删除 '建议补充内容' 占位章
- cmake: arm-none-eabi-gcc.cmake 工具链找不到时 FATAL_ERROR 而非静默

cmake/arm-none-eabi-gcc.cmake

@@ -26,6 +26,10 @@ set(CMAKE_C_FLAGS_RELEASE "")
 set(CMAKE_CXX_FLAGS_RELEASE "")
 set(CMAKE_ASM_FLAGS_RELEASE "")
 
+if(NOT TOOLCHAIN_DIRECTORY)
+    message(FATAL_ERROR "Cannot find ARM GCC toolchain. Set TOOLCHAIN_DIRECTORY cache variable or install toolchain in one of the default locations.")
+endif()
+
 set(CMAKE_COLOR_DIAGNOSTICS ON)
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON CACHE INTERNAL "")