docs: Add node.js embedding gum example

examples/gum.js

@@ -0,0 +1,13 @@
+const { spawn } = require("child_process");
+
+const activities = ["walking", "running", "cycling", "driving", "transport"];
+
+console.log("What's your favorite activity?")
+const gum = spawn("gum", ["choose", ...activities]);
+
+gum.stderr.pipe(process.stderr);
+
+gum.stdout.on("data", data => {
+    const activity = data.toString().trim();
+    console.log(`I like ${activity} too!`);
+});