Refactor out legacy content

[IanMayo]

• [x] create placeholder for legacy mock content /data-legacy
• [x] include readme in legacy folder, explaining where content came from
• [x] move non-British regions to legacy folder
• [x] move non-complex categories to legacy folder
• [x] remove britain-complex content that doesn't follow page model
• [x] fix quick-links to avoid missing pages
• [x] create single non-complex category (to check we are walking multiple categories
• [x] copy (fixed) britain-complex content to britain-legacy
• [x] rename britain-legacy content to make it unique
• [x] expect data folder to contain these folders:
  • [ ] britain1
  • [ ] britain_complx
  • [ ] britain_legacy
  • [ ] introduction
  • [ ] PlatformData
  • [ ] QuickLinksData
• [ ] check above runs through dita converter (update from Robin's branch)

[IanMayo]

Done.