Simplify and improve the 404 page

This removes the old `404.html` file (which Jekyll overwrote with the
contents of `404.md` anyway), and properly uses the `page` layout that
the old `404.md` was shoehorning into the content area.

With the `page` layout in effect, the sidebar has been modified not to
list the 404 page in the site contents.

404.md

@@ -1,9 +1,8 @@
 ---
-layout: default
+layout: page
 title: "404: Page not found"
 permalink: 404.html
 ---
 
-# 404: Page not found
-
-Sorry, we've misplaced that URL or it's pointing to something that doesn't exist. [Head back home]({{ site.baseurl }}) to try finding it again.
+Sorry, we've misplaced that URL or it's pointing to something that doesn't exist.
+[Head back home]({{ site.baseurl }}) to try finding it again.

_includes/sidebar.html

@@ -20,7 +20,7 @@
 
     {% assign pages_list = site.pages | sort:"url" %}
     {% for node in pages_list %}
-      {% if node.title != null %}
+      {% if node.title != null and node.url != "/404.html" %}
         {% if node.layout == "page" %}
           <a class="sidebar-nav-item{% if page.url == node.url %} active{% endif %}" href="{{ site.baseurl }}{{ node.url }}">{{ node.title }}</a>
         {% endif %}