Trigger condition improvement in github actions

[idubnori]

In order to resolve docker pull issue the below, I think should change the trigger condition described in the document. docker pull will still occur even if exclude synchronize inside the pr-agent codebase.

-every time you run GitHub action, you pull a docker. This operation costs us money, and with large numbers, it's not a negligible amount. 'synchronize' will force us to use far more expensive docker hosting solutions.
#648 (comment)

Suggesions

• Add the pull_request types into the github actions workflow docs based on the code base to suppress pr-agent docker image pull from being executed in synchronize event. Please note that the default pull request types are

  By default, a workflow only runs when a pull_request event's activity type is opened, synchronize, or reopened
  https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request

• Add else condition into this and logging why pr-agent does not execute. Users who are configuring github actions according to the current documentation, the reason why pr-agent is not executed when the synchronize event occurs is confusing and looks like a bug.