Add some extra details to webhook docs

Update the webhook JSON payloads to real ones,
and show there is a difference between an automated build webhook payload and a normal repo payload

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

Signed-off-by: Sven Dowideit <SvenDowideit@docker.com>
i worry that as the feature set for automated builds is not the same as for the other type of repo, that we need to separate them.

if all repo operations can be done on automated repo's (like pushing to them too), then the crosslinking is safer.