How to export to top level of package?

# MWE

```sh
$ mkdir example
$ cd example
$ nbdev_new
$ cd nbs
$ code 00_core.ipynb
```

How can I use `default_exp` to instead of exporting to `example.core` I can export everything to the top level of the package so I can just go `from example import ...`

Start with the `default_exp` handling used from `nbs/00_core.ipynb` after creating a project with `nbdev_new`. Check whether exporting directly to `example` is supported, then document the setting and verify the resulting `from example import ...` usage, or clarify the limitation if it is not.

Written by the indexing model from the issue text.