Expand documentation on triggers

Currently the official docs don't explain how triggers can be fired as well via API. The only reference that can be found online is a blog from 2017: https://www.raymondcamden.com/2017/02/02/working-with-openwhisk-triggers

Locate the official trigger documentation in the OpenServerless site and compare it with the linked 2017 blog post, since no documentation file is named in the issue. Document how triggers can be fired through the API, with an example that lets readers verify the behavior.

Written by the indexing model from the issue text.