CSS code samples

[simoneduca]

Add styles alongside HTML and njk on the site's examples.

[michaelcattell]

Originally mocked up the order:
HTML | Nunjucks | CSS

But Martin asked:
one thought - perhaps better to have the tab order be HTML | CSS | Nunjucks, as people will either need (HTML and CSS) or (Nunjucks)

So the order will be HTML | CSS | Nunjucks :

And the select state for the css:

Behaviours like the other tabs (ignore the css, ripped it from dwp examples site for random)
CSS select state shows the css down below (again, ignore the colour the css class is - unsure what colour we are to use but I just did a blue for no reason)

[martinwake]

Related to #303