feat: add custom 404 page

Author: dayatnhbtc

src/pages/404.astro

@@ -0,0 +1,17 @@
+---
+import Base from "../layouts/Base.astro";
+---
+
+<Base title="404 — Not Found" description="The page you’re looking for doesn’t exist.">
+  <div class="wrap">
+    <nav aria-label="Primary">
+      <a href="/">About</a> /
+      <a href="/blog">Blog</a>
+    </nav>
+
+    <h1>Page not found</h1>
+    <p class="muted">The link may be broken or the page may have moved.</p>
+    <p><a href="/">← Back to home</a></p>
+  </div>
+</Base>
+