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>
Show image of the Build trigger screen, and add a little search engine fodder for it.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #9464 from SvenDowideit/sven_tries_webhooks

Add some extra details to webhook docs