Populate or remove Discussion extra page

[tobyhodges]

At the moment, the Discussion page under Extras is blank. It should either be removed, or populated with relevant discussion of the lesson content and design. Examples of what could be included in the page are:

• an overview of the reasoning behind the choice of image viewer used to fix Use a different image viewer #134
• a discussion of the different example images used in the lesson and why those images/formats were chosen