Add comprehensive documentation for SnapForm extension

- Created development.md for development setup and licensing information.
- Added features.md detailing functionalities and usage instructions for developers and users.
- Introduced index.md to serve as a navigation hub for the documentation.
- Developed outputs.md to explain output formats and their specifications.
- Established service.md to describe the AI Service architecture and environment setup.
- Created surface.md to outline the process of converting Image Empty to perspective meshes.

docs/index.md

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+# SnapForm 文档
+
+技术内容已按主题拆分：
+
+- [功能与使用](features.md)：能力概览、安装使用和 Image Empty 去背景
+- [Depth Plane](depth-plane.md)：深度面片的数据、节点与材质流程
+- [Surface](surface.md)：透视网格、Geometry Nodes 后处理与材质法线
+- [输出格式](outputs.md)：Debug 输出类型、参数和任务产物
+- [AI Service](service.md)：Environment、模型缓存、任务目录与运行机制
+- [开发与许可证](development.md)：开发验证、打包、上游版本与许可证