site: vary on Accept via vercel.json so caches don't cross-pollute

Middleware sets `Vary: Accept` but Next's prerender cache strips it on the way out, even on Vercel. Vercel's platform-level headers config applies after the cache, so the header makes it onto every response.

Plugs the upstream-proxy concern noted in #623: a cache between the client and Vercel could otherwise key only on URL and serve a markdown response back to a browser (or vice versa).

Author: rgbkrk

vercel.json

@@ -0,0 +1,16 @@
+{
+  "headers": [
+    {
+      "source": "/",
+      "headers": [{ "key": "Vary", "value": "Accept" }]
+    },
+    {
+      "source": "/blog",
+      "headers": [{ "key": "Vary", "value": "Accept" }]
+    },
+    {
+      "source": "/blog/:slug",
+      "headers": [{ "key": "Vary", "value": "Accept" }]
+    }
+  ]
+}