Add a recap and shortcode for including md files

[davidwilby]

This PR adds a shortcode layouts/shortcodes/include.html which allows us to include other pages.

This is motivated by the desire to have some recap materials, but I wanted to avoid repetition (like a good little software engineer).

Interested to know whether people think this is a good approach or whether we'd just prefer something simpler.

Resolves #52

[davidwilby]

Thanks Neil :+1:

I'm not very familiar with Jekyll so only loosely understand how its working but can see the page renders correctly in the preview.

Me neither, I got it from a forum..