Add archive gallery.

layouts/404.html

@@ -0,0 +1,34 @@
+{{ define "main" }}
+<div class="container">
+  <div class="row">
+    <div class="col mt-5 mb-5">
+      {{ with resources.Get "svg/404.svg" }}
+        {{ .Content | safeHTML }}
+      {{ else }}
+        <h1 class="display-1 mx-auto text-center">
+          404
+        </h1>
+      {{ end }}
+    </div>
+  </div>
+  <div class="row">
+    <div class="col">
+      <h4>
+        Page not found
+      </h4>
+      <p>
+        The page you were trying to navigate to was not found on this
+        server.
+      </p>
+      <p class="fw-bold">
+        Please not that the old XLToolbox.net site
+        has been retired. Navigate to the homepage to read more about this.
+      </p>
+      <p>
+        <a href="{{ .Site.BaseURL }}">Back to the home page.</a>
+      </p>
+    </div>
+  </div>
+</div>
+{{ end }}
+