Add a tiny end-to-end example to README/docs

I was looking through Spec Kit and the tagline “Define what to build before building it — with any AI coding agent” makes the goal clear, but I had a little trouble forming a mental model of the actual workflow from the README alone.

It would help newcomers to include a very small example near the Get Started section: a short spec file or PRD fragment, the command/template used to process it, and what the next artifact looks like when handed to an agent like Copilot. Even a contrived example like “add a /health endpoint” would make the boundaries clearer.

If a fuller example already lives in docs or an examples directory, a direct link from the README would still be useful. The main benefit is reducing the blank-page problem for people trying to map spec-driven development onto their own repo.

从 README 的 Get Started 部分开始，检查文档或 examples 目录中是否已经存在更完整的示例。添加一个小型端到端示例，涵盖 spec 或 PRD 片段、所使用的 command 或 template，以及为 AI coding agent 生成的 artifact；或者链接到现有示例。当新手无需面对 blank-page problem 就能按照 workflow 操作时，即视为完成。

由索引模型根据 Issue 内容生成。