Improve component previews in the docs

[jhackett1]

fixed position components don't look good right now, and margins between previews and code samples could do some work.

eg. https://design-system.hackney.gov.uk/components/back-to-top

[jhackett1]

i reckon the fix is a component like <PreviewArea> or similar which:

• initialises javascript for its children
• acts as the viewport for position: fixed elements inside, perhaps with a minimum height
• has sensible vertical margins to avoid bumping up against other stuff on the page

now, how to tackle the horizontal container padding issue?

[jhackett1]

horizontal padding tutned out to be fixable by adjusting $govuk-page-width value