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 found "imagehash" confusing - though y, this eg is one where I tagged an existing image, I'll add one