Docs: Missing payload sample in documentation

### Is your proposal related to a problem?

When we need to use the one-off scheduled events/cron trigger need to trigger it once to see the arguments we are getting from Hasura. For event triggers, we have documentation that describes how the JSON payload would look like
https://hasura.io/docs/1.0/graphql/core/event-triggers/payload.html#json-payload

### Describe the solution you'd like

Need section/page which shows the sample JSON payload.

### Describe alternatives you've considered

Users need to use httpbin / webhook to know the payload.
The purpose of adding this in the doc is to speed up the development

### If the feature is approved, would you be willing to submit a PR?

Yes