When generating index.ipynb, cell content is not sanitized

# Steps to reproduce
1. Create a repository with " in description, like this one: https://github.com/ozpau/nbdev_new_cant_handle_this
2. Clone, cd into it, and call `nbdev_new`
3. This will generate invalid `index.ipynb`: https://github.com/ozpau/nbdev_new_cant_handle_this/blob/main/nbs/index.ipynb?short_path=7a1e639#L19

Notice that double quotation marks that breaks the json.