[add] Dev Container configuration for auto CDE

[Copilot]

Adds Dev Container configuration following the pattern from idea2app/Deno-quick-start.

Changes

• .devcontainer/Dockerfile: Microsoft C++ devcontainer base image with XMake installed via official script
• .devcontainer/devcontainer.json: VS Code extensions for C++ development (all recommended extensions from .vscode/extensions.json)
• ReadMe.md: Simplified cloud development documentation since XMake is now pre-installed in the container

This enables one-click development environment setup via GitHub Codespaces or any Dev Container-compatible IDE.

Original prompt

参考 https://github.com/idea2app/Deno-quick-start 增加 Dev Container 配置。

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[TechQuery]

@copilot 参考项目的文档中关于云开发环境中使用本模板仓库的说明也要照抄。

[Copilot]

@copilot 参考项目的文档中关于云开发环境中使用本模板仓库的说明也要照抄。

Updated the "Use this template project in Cloud" section to remove the manual XMake installation step since it's now pre-installed in the Dev Container (1f2f6b3).