Correct people pluralisation rules in en.yml
Make policies translatable
Add known Welsh translations
Update locale files with new keys
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 #822 from alphagov/46221595-translate-policies

Make policies translatable

https://www.pivotaltracker.com/story/show/46221595
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