Add placeholder pages for docs site

docs/404.html

@@ -1,6 +1,7 @@
 ---
 permalink: /404.html
 layout: default
+nav_exclude: true
 ---
 
 <style type="text/css" media="screen">
@@ -9,6 +10,7 @@ layout: default
     max-width: 600px;
     text-align: center;
   }
+
   h1 {
     margin: 30px 0;
     font-size: 4em;
@@ -22,4 +24,4 @@ layout: default
 
   <p><strong>Page not found :(</strong></p>
   <p>The requested page could not be found.</p>
-</div>
+</div>