docs: Add a not found page

src/doc/not_found.md

@@ -0,0 +1,20 @@
+% Not Found
+
+<!-- Completely hide the TOC and the section numbers -->
+<style type="text/css">
+#TOC { display: none; }
+.header-section-number { display: none; }
+li {list-style-type: none; }
+</style>
+
+Looks like you've taken a wrong turn.
+
+Some things that might be helpful to you though:
+
+## Reference
+* [The Rust official site](http://rust-lang.org)
+* [The Rust reference manual](http://static.rust-lang.org/doc/master/rust.html) (* [PDF](http://static.rust-lang.org/doc/master/rust.pdf))
+
+## Docs
+* [The standard library (stable)](http://doc.rust-lang.org/doc/0.10/std/index.html)
+* [The standard library (master)](http://doc.rust-lang.org/doc/master/std/index.html)