Update repo hooks documentation

I ran into some confusion over the weekend thinking that the [web service should listen to all events by default](https://github.com/github/github-services/pull/712). @kdaigle suggested that I update the docs, and so I have.

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Clarify and expand hook documentation to explain the differences between a `service` and an `event`, each of which got a section header with links throughout. Provide details on service configuration settings, default events and supported events.

* Increase heading levels to support new sections
* Provide more linking throughout
* Add examples in the service and event sections