add 404 page (#132)

Co-authored-by: Sean Killeen <SeanKilleen@gmail.com>

404.html

@@ -0,0 +1,14 @@
+---
+layout: default
+permalink: /404.html
+---
+
+<div class="container not-found-page">
+    <div class="content">
+        <div class="text-center">
+            <h1>404 page</h1>
+            <a class="btn btn-danger btn-lg" href="/" role="button">Back to home</a>
+        </div>
+    </div>
+</div>
+

css/main.scss

@@ -198,3 +198,9 @@ ul.across li { float: left; display: block; width: 9em }
 .footer a {
     color: #EB4D44;
 }
+
+.not-found-page {
+    justify-content: center;
+    display: flex;
+    align-items: center;
+}