Add 404 template

rainfall/patterns/hidden-404.php

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+<?php
+/**
+ * Title: 404 content
+ * Slug: rainfall/hidden-404
+ * Categories: hidden
+ * Inserter: no
+ */
+?>
+<!-- wp:heading {"align":"full","level":1,"backgroundColor":"light-orange"} -->
+<h1 class="alignfull has-light-orange-background-color has-background"><?php esc_html_e( 'Missing page', 'rainfall' ); ?></h1>
+<!-- /wp:heading -->
+
+<!-- wp:paragraph -->
+<p><?php esc_html_e( 'The page you requested cannot be found; poor luck, eh?', 'rainfall' ); ?></p>
+<!-- /wp:paragraph -->

rainfall/templates/404.html

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+<!-- wp:template-part {"slug":"header","theme":"rainfall"} /-->
+
+<!-- wp:group {"layout":{"inherit":true},"tagName":"main"} -->
+<main class="wp-block-group">
+    <!-- wp:pattern {"slug":"rainfall/hidden-404"} /-->
+</main>
+<!-- /wp:group -->
+
+<!-- wp:template-part {"slug":"footer","theme":"rainfall"} /-->