[Documentation] The reference template

[alcarney]

We need to have a document giving an overview of the reference template file (_layout/reference.html) including:

• What elements it defines on the page (tutorial links etc.)
• How the 'We need you!' poster is activated
• How the getting started and general tutorial links are generated
• The important CSS classes defined for theme designers

Note: You may assume the reader is familiar with basic HTML and Liquid syntax