Fix 404 page missing from site (#5707)

After
faa3ca3,
the 404 page was generating but being placed in a folder instead of in
the root directory, causing it to not be found.

Before fix: https://dart.dev/parker-is-cool
After fix:
https://dart-dev--pr5707-fix-404-missing-9rfsj1qx.web.app/parker-is-cool

Author: parlough

src/content/404.html

@@ -3,7 +3,9 @@
 title: "404: Page not found"
 description: "dart.dev's 404 page."
 sitemap: false
+permalink: /404.html
 ---
+
 <div class="text-center">
   <h1>404</h1>
   <h2>Page not found</h2>