Review CMS 6 changelog and docs prior to stable release

[GuySartorelli]

We should give the CMS 6 changelog a thorough review before we produce the stable release

Acceptance Criteria

• Review structure of changelog and move things around as necessary to provide a clear flow
  • e.g. maybe have non-breaking changes first
  • e.g. maybe group the validation changes together
• Ensure the automated list of changes is accurate
• Ensure instructions are clear
• Remove anything that's unnecessary
• Make sure we've deleted all the CMS 5 changelogs
• Final check for renamed classes e.g. ViewableData should be ModelData