Renaming sections to section_groups

In both document and manual presenters, section_groups were using the
top level sections array from the JSON.
Linking to the sections from Manuals and Documents

This work by introducing a path method to section, which takes the
parents to construct the path for HMRC manuals
Renaming top level sections in placeholders

For the JSON, rename sections as section groups in order to match up
with the response being returned from the content api. Also adding slugs
to the sections in the placeholder HMRC manual.
Merge pull request #41 from alphagov/display-manual-sections

Display manual sections