Redirect old getting started URL (elixir-lang#773)

The Pragmatic "Programming Elixir" book references
`http://elixir-lang.org/getting_started/1.html` which has been moved to
`http://elixir-lang.org/getting-started/introduction.html`. This PR adds
a `redirect_from` on the `introduction.markdown` page to ensure that
readers of that book are able to use the URL in the book without getting
a 404.

Author: conroywhitney

getting-started/introduction.markdown

@@ -1,6 +1,7 @@
 ---
 layout: getting-started
 title: Introduction
+redirect_from: /getting_started/1.html
 ---
 
 # {{ page.title }}