I'm not sure how it usually goes but I believe writing a whitepaper or roadmap can be done while you work on your website, especially if you work together with other people. It will also help people to consider your idea more seriously since they can see what is your plan in the future, although it doesn't guarantee that you will succeed. Unless you work alone, I don't see the point of delaying of writing or thinking about your roadmap unless you want to just go day by day with your projects.