Include contents on doc series page
Give group headings some padding

This gives the headings a bit of breathing space when the user jumps
to the anchor via the new contents links.
Merge pull request #824 from alphagov/56260002-contents-on-doc-series

Add secondary nav in the margin of doc series page, linking to the groups in the series.

https://www.pivotaltracker.com/story/show/56260002