feat(404): add custom 404 page

Add a custom 404 page with a title, layout, and content explaining that the requested page couldn't be found. Include a link to the home page.

404.html

@@ -0,0 +1,11 @@
+---
+layout: default
+title: 404
+permalink: /404
+nav_exclude: true
+search_exclude: true
+---
+
+<h1>Page not found</h1>
+
+<p>The page you requested could not be found. Try using the navigation {% if site.search_enabled != false %}or search {% endif %}to find what you're looking for or go to this <a href="{{ '/' | absolute_url }}">site's home page</a>.</p>