Reorganise release notes

* Fixed a few minor wording issues
* Removed the overview section. I think the "What's new" headlines do a
  good job, and the existing content wasn't really suitable to give a
  nice overview.
* Reduced importance of "Customisation got easier" and "shipping app"
  by moving it further down the list. There's more exciting changes
  in this release, and nobody gets excited about us breaking backwards
  compatibility...
* Add release notes for search changes.
* Drop a random list of explaining product structure, probably a
  copy-paste error
* Moved "cleanup around shipping methods" section into breaking changes,
  because really they are. Also removed a duplicate entry, and the entry
  on the since removed upper_limit field.