add link rel=canonical

Author: mbostock

observablehq.config.ts

@@ -104,7 +104,8 @@ export default {
   globalStylesheets: [
     "https://fonts.googleapis.com/css2?family=Inter:ital,opsz,wght@0,14..32,100..900;1,14..32,100..900&family=Spline+Sans+Mono:ital,wght@0,300..700;1,300..700&display=swap"
   ],
-  head: `<link rel="apple-touch-icon" href="/observable.png">
+  head: ({path}) => `<link rel="canonical" href="https://observablehq.com/framework${path.replace(/\/index$/, "/")}">
+<link rel="apple-touch-icon" href="/observable.png">
 <link rel="icon" type="image/png" href="/observable.png" sizes="32x32">${
     process.env.CI
       ? `