Describe file structure created by `nbdev_new`

Inspired by [VSCode's extensions docs](https://code.visualstudio.com/api/get-started/extension-anatomy):

Here's the structure created by `nbdev_new` as of writing:

```sh
$ tree -a -I .git -I .quarto
.
├── .github
│   └── workflows
│   ├── deploy.yaml
│   └── test.yaml
├── .gitignore
├── 00_core.ipynb
├── LICENSE
├── MANIFEST.in
├── README.md
├── _docs
│   ├── robots.txt
│   └── sitemap.xml
├── _quarto.yml
├── index.ipynb
├── settings.ini
├── setup.py
├── styles.css
└── {lib_path}
├── __init__.py
├── _modidx.py
└── core.py
```

从 `nbdev_new` 命令开始，将其输出与 issue 中显示的目录树进行比较。记录生成的 `.github/workflows`、notebook、配置文件、`_docs` 和 `{lib_path}` 内容，并使描述与当前输出保持一致。

由索引模型根据 Issue 内容生成。