fix(404): noindex the error page instead of emitting a canonical to /404/

A canonical pointing at a URL that does not exist is worse than none. Normal pages
are untouched (canonical still emitted, no robots meta).

src/pages/404.astro

@@ -15,6 +15,7 @@ const fmt = (d: Date) =>
 <BaseLayout
   title="404 — Page Not Found — Mr Hoelee"
   description="That URL does not exist on this site. Search the posts or browse the archive."
+  noindex
 >
   <h1>404 — container exited (code 1)</h1>
   <p>