Add SPA section to docsite

Prettynice is not built _for_ SPAs, but you can still build a SPA with it, and the architecture gives you some nice benefits over a traditional, serverless SPA. Your `server/` can serve your backend API, and route all other requests to a response that loads your SPA from `client/src/components/App.gren`. Build `App` in the `client` folder the same way you would build any other Gren SPA.