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

[stefanheyder]

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.

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

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.