Update repo hooks documentation

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
Update hooks.md with editorial changes

Pedants rejoice!
One last update to hooks.md

Plurals are goods.
Merge pull request #337 from elskwid/patch-1

Update repo hooks documentation