I haven't tried to used Prismic Preview yet but before I try it, I want to know if it fits my use case. Does it only allow a user to update the content of a field that is already on the live site? Or is it capable of rendering new content on the site? For example, could the user add a new slice to the page and have it render in the preview?
As long as the addition of this new field or Slice is reflected in your Gatsby application. So yes, users can add new content and documents using the Previews.
Let me know if you have any further questions about this.