Publish manuals, sections and change notes to the Publishing API

The Publishing API will eventually replace the Content API.

- [x] publish manualsto the Publishing API.
- [x] publish manual sections to the Publishing API.
- [x] publish change notes to the Publishing API.
- [x] augment the Publishing API payload with organisation info (this needs to be looked up against the Organisations API)
Should this always be `major`, or should it be something like `sections.all?(&:minor_update?) ? "minor" : "major"`?