added default 404 layout

assets/sass/main.scss

@@ -130,6 +130,13 @@ section.article-list {
 	gap: 40px;
 }
 
+.center-box {
+	display: flex;
+	flex-direction: column;
+	justify-content: center;
+	align-items: center;
+}
+
 .fill-wh {
 	width: 100%;
 	height: 100%

layouts/404.html

@@ -0,0 +1,7 @@
+{{ define "main" }}
+<div class="center-box">
+	<h1>404</h1>
+	<h2>This page doesn't exist. Let's go <a href="/">home</a></h2>
+</div>
+{{ end }}
+