When generating index.ipynb, cell content is not sanitized

[ozpau]

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.

[ozpau]

settings.ini seems fine.

This repo https://github.com/ozpau/nbdev_new_test with sfdasd\n\n\n\n\n\nkeywords = one two three " }] in description doesn't break it.

[ozpau]

Generation of nbs/nbdev.yml is also not sanitized: https://github.com/ozpau/nbdev_new_cant_handle_this/blob/main/nbs/nbdev.yml#L7