docs: fix rewrites for markdown and plain content (#13916)

www/apps/resources/next.config.mjs

@@ -298,23 +298,12 @@ const nextConfig = {
           has: [
             {
               type: "header",
-              key: "accept",
-              value: "text/markdown",
+              key: "Accept",
+              value: ".*(text/markdown|text/plain).*",
             },
           ],
           destination: "/md-content/:path*",
         },
-        {
-          source: "/:path*",
-          destination: "/md-content/:path*",
-          has: [
-            {
-              type: "header",
-              key: "accept",
-              value: "text/plain",
-            },
-          ],
-        },
       ],
     }
   },