Add playground-redirect.html back to avoid HTTP 404

website/static/playground-redirect.html

@@ -0,0 +1,36 @@
+<!doctype html>
+<html>
+  <head>
+    <meta charset="utf-8" />
+    <title>Redirecting…</title>
+  </head>
+  <body>
+    <h1>Redirecting…</h1>
+    <p>
+      Redirecting you to the appropriate playground. If the redirect doesn’t
+      work, <a href="javascript:history.back()">go back to the PR</a> and click
+      the appropriate link:<br />
+      <img
+        src="https://cdn.netlify.com/f0b93f3003d5669924b6e73540bbfe07bc3d8d95/34c5d/img/blog/deploy-preview-workflow.gif"
+        alt="click on the link next to 'deploy/netlify'"
+      />
+    </p>
+    <script>
+      const match =
+        /^https:\/\/github\.com\/prettier\/prettier\/pull\/(\d+)/.exec(
+          document.referrer,
+        );
+      if (match != null) {
+        const [, /* url */ pr] = match;
+        location.replace(
+          `https://deploy-preview-${pr}--prettier.netlify.app/playground`,
+        );
+      } else {
+        const el = document.createElement("p");
+        el.textContent = `The referrer URL (${document.referrer}) was not recognized. Sorry :(`;
+        document.body.appendChild(el);
+      }
+    </script>
+  </body>
+</html>
+<!-- TODO: Remove this once PRs not linked to this page -->