Improve Contacts API documentation.

@DavidHuie @arionhardison [Trello card](https://trello.com/c/UgWzNaov/482-add-read-access-to-contact-timestamps).

* Add created_at field to the contact resource and examples.
* Move the resource description to the same page.
* Update the page to use similar structure to what we have on the Donations and People API.