nbdev_preview strips leading digits from directories in `sidebar.yml`, but not in `_docs`

As the title suggests, calling `nbdev_preview` with a file inside a directory whose name starts with leading digits (e.g. `00_test`) will result in broken documentation.

## steps to reproduce
Create a new nbdev_project and a `00_test` directory under notebooks.

```bash
nbdev_init
mkdir nbs/00_test
mv nbs/00_core.ipynb nbs/00_test/00_core.ipynb
nbdev_preview
```

will result in the following `sidebar.yml`

```yaml
website:
sidebar:
contents:
- index.ipynb
- section: test
contents:
- test/00_core.ipynb
```

which is broken, as the correct path would be `00_test/00_core.ipynb` (file is under `_proc/00_test/00_core.html.md`). Thus the link in the preview will not work.

Riproduci il problema con nbdev_init, una directory nbs/00_test e nbdev_preview. Confronta il percorso sidebar.yml generato con il percorso _proc/00_test/00_core.html.md generato; completato significa che le cifre iniziali vengono preservate, così che il link di anteprima venga risolto correttamente.

Scritto dal modello di indicizzazione a partire dal testo della issue.