Allow returning static content in Doc in RPC

[Jand42]

Needs a WebSharper feature to enable this, see dotnet-websharper/core#1065.

Sending Doc's from the client should throw a compile-time error. Any non-serializable content sent from the server should throw a server-side error while running the RPC call instead of returning an empty object.

See a use case at https://stackoverflow.com/questions/58052959/using-websharper-to-render-svgs-server-side/58054334