Add 404 page

404.html

@@ -0,0 +1,27 @@
+---
+title: Page not found - Minetest
+---
+
+{% include header.html %}
+
+<div class="jumbotron reduced credits">
+  <div class="center">
+    <h1>Page not found :(</h1>
+  </div>
+</div>
+
+<div class="content">
+  <img style="float:left;margin: -10px 40px 0 0;"
+    src="media/unknown_node.jpg"
+    alt="unknown_node">
+
+  <h2>Sorry, we could not find that page!</h2>
+
+  <p>
+    Please report any broken links on the
+    <a href="https://github.com/minetest/minetest.github.io/issues">issues page
+        for this website</a>.
+  </p>
+</div>
+
+{% include footer.html %}