Add homepage illustration (and other tweaks)

[paulrobertlloyd]

Adds homepage illustration, Open Graph share image, and reviews content such that:

• code examples are correctly indented
• pages are listed in the sub navigation in the same order as the homepage
• product name is capitalised (to match other X-GOVUK products)
• correct heading styles are used on the homepage

Also added a .editorconfig file. Closes #9. 💅

[frankieroberto]

Thanks for the great illustration and all the small fixes! 🙌

Left a couple of minor comments.