fix(nginx): return a real 404 instead of a 500 redirect loop

The 404 fallback pointed at /404/index.html, but Astro emits 404.html,
so every unknown URL looped internally and nginx answered 500. The
try_files fallback also served the error page with a 200 status.
Fall through to =404 and point error_page at /404.html.

deploy/nginx.conf

@@ -14,10 +14,10 @@ server {
   }
 
   location / {
-    try_files $uri $uri/ /404/index.html;
+    try_files $uri $uri/ =404;
   }
 
-  error_page 404 /404/index.html;
+  error_page 404 /404.html;
 
   location ~* \.(?:css|js|jpg|jpeg|png|webp|svg|ico|woff2)$ {
     try_files $uri =404;