[TO_DISCUSS] Publish manuals to the Publishing API

The Publishing API will eventually replace the Content API.

Before merging: 
- [ ] publish manual sections to the Publishing API.
- [ ] publish change notes to the Publishing API.
- [ ] augment the Publishing API payload with organisation info (this needs to be looked up against the Organisations API)
Given this is the only place the `publishing_api_exporter` is used currently, can we concrete it here rather than in the wiring, given that we're aiming to murder the wiring with extreme prejudice? :)

```ruby
def publishing_api_exporter
  ->(manual) {
    ManualPublishingAPIExporter.new(publishing_api, manual).call
  }
end

def publishing_api
  GdsApi::PublishingApi.new(Plek.new.find("publishing-api"))
end
```