Add SPA section to docsite

[blaix]

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.