Port guidance from govuk prototype kit about passing data

[vickytnz]

There are several pages from the govuk prototype kit guidance

Basic usage

• Pass data from page to page

Advanced usage

• Create routes (server side programming)
• Branching journeys including branching using radios and branching journeys using checkboxes
• Store data in session

This is also a chance to consolidate some guidance as there may be repetition and overlap with the basic tutorial.