Miki/add donor_id to the Donations API documentation

@DavidHuie @arionhardison [Trello card](https://trello.com/c/yiNP8erp/459-writeable-donor-id-in-donations-api)

* add `donor_id` to the resource description and the sample jsons
* rewrite the create endpoint example to use `donor_id` and add guidelines about how to create or look up a donor
* use a donation field and not a person one in the update endpoint example
* remove optional `tracking_code_slug` field from the create endpoint example