Add cookbook section + layout

Adding a cookbook is fairly easy:

1. Add a new file in `cookbook/` directory, such as:

    cookbook/how-to-do-that.markdown

2. Write at least the following configuration block:

    ---
    layout: cookbook
    title:  How To Do That
    ---

3. Add a link in the `index.markdown` file:

    * [How To Do That](cookbook/how-to-do-that.html)

You are done!