feat: add Vercel Web Analytics to docs site

The docs are served from checklyhq.com/docs via a Vercel rewrite to
Mintlify, so the marketing site's <Analytics /> component never runs on
these pages — Vercel proxies Mintlify's HTML and our React tree is never
involved.

Since docs share an origin with the marketing site, the insights script
already served at /_vercel/insights/script.js loads and beacons back to
/_vercel/insights/view with correct attribution. Mintlify has no Vercel
analytics integration, so this goes in via a custom script.

The absolute path matters: /docs/_vercel/insights/script.js 404s, only
the root path resolves.

Verified against production: initial pageview fires, and client-side
sidebar navigation registers as a distinct path rather than collapsing
into the entry page (the script patches history.pushState and listens
for popstate).

Author: Dgiordano33

docs.json

@@ -22,6 +22,9 @@
     },
     {
       "src": "/unify-tracking.js"
+    },
+    {
+      "src": "/vercel-analytics.js"
     }
   ],
   "integrations": {

vercel-analytics.js

@@ -0,0 +1,12 @@
+// Vercel Web Analytics
+// These docs are served under checklyhq.com/docs via a Vercel rewrite, so the
+// insights script and its beacon only resolve at the domain root. A path
+// relative to /docs 404s.
+(function () {
+  if (window.__vercelInsightsAdded) return;
+  window.__vercelInsightsAdded = true;
+
+  var script = document.createElement('script');
+  script.src = '/_vercel/insights/script.js';
+  document.head.appendChild(script);
+})();