Added new ant target docs: "Generate documentation"

```
New ant target is added to build.xml
name: "docs"
description: "Generate documentation"
```

Original issue reported on code.google.com by `natry.cl...@gmail.com` on 20 Sep 2013 at 10:32

Attachments:
- [build.xml.patch](https://storage.googleapis.com/google-code-attachments/s2-geometry-library-java/issue-2/comment-0/build.xml.patch)

打开 build.xml，检查随附的 build.xml.patch 中提议的 Ant target。运行现有的文档生成命令或 target，然后验证 `ant docs` 是否以名称 "docs" 提供并生成项目文档。

由索引模型根据 Issue 内容生成。