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
```

Start at the `nbdev_new` command and compare its output with the tree shown in the issue. Document the generated `.github/workflows`, notebooks, configuration files, `_docs`, and `{lib_path}` contents, keeping the description aligned with the current output.

Written by the indexing model from the issue text.