chore: Place pagefind assets next to other assets

This allows us to match requests to pagefind URLs using our
asset rate limiting rule instead of the catch-all rule. The
pagefind assets were previously /pagefind. Now, they are placed
alongside other assets under /_/pagefind.

Author: Techassi

package.json

@@ -13,6 +13,6 @@
     "ui"
   ],
   "scripts": {
-    "build-search-index": "npx pagefind --site build/site"
+    "build-search-index": "npx pagefind --site build/site --output-subdir _/pagefind"
   }
 }