[WIP] Organise docs in a document series in groups

I'm submitting this pull request so people can start to take a look at it before I've finished, so we can avoid end-loading review of the biggest story in the iteration.

I'm not in tomorrow (Friday the 30th) and delaying a first-pass review until Monday feels rather risky.

The core functionality is done; I've just got a few things to tidy up (including cucumber). See the task list in the story for the full list.

Story: https://www.pivotaltracker.com/story/show/52472099