feat: add 404 not found page

Add a new 404.astro page, to provide a user-friendly error page when routes are not found.

src/pages/404.astro

@@ -0,0 +1,21 @@
+---
+import { css } from '@styled-system/css'
+import Basic from '@/components/layout/Basic.astro'
+
+const headingClass = css({
+  fontSize: '50px',
+  textAlign: 'center'
+})
+const notFoundDescription = css({
+  fontSize: 'lg',
+  textAlign: 'center'
+})
+---
+
+<Basic title="Not Found">
+  <h1 class={headingClass}>404 Not Found</h1>
+  <p class={notFoundDescription}>
+    The page you are looking for does not exist or has been moved. Try going
+    back to the <a href="/">home page</a>.
+  </p>
+</Basic>