docs: set-up sapper template

docs/src/server.js

@@ -0,0 +1,12 @@
+import sirv from "sirv";
+import polka from "polka";
+import * as sapper from "@sapper/server";
+
+const { PORT, NODE_ENV } = process.env;
+const dev = NODE_ENV === "development";
+
+polka()
+  .use(sirv("static", { dev }), sapper.middleware())
+  .listen(PORT, (err) => {
+    if (err) console.log("error", err);
+  });