Document which template context variables are available for automation actions

[chrisguidry]

Our current documentation for Jinja templating in automation actions only mentions the dynamic context variables (like flow_run, work_pool, etc), but these variables are shortcuts to pull objects from the API and can be problematic due to the timing gaps between the original event and when the action runs (like if the flow run has changed states since the automation was triggered, etc).

https://docs-3.prefect.io/3.0rc/react/understand-events/automations-triggers#templating-with-jinja

We should start with the standard variables:

{
    "automation": triggered_action.automation,
    "event": triggered_action.triggering_event,
    "labels": LabelDiver(triggered_action.triggering_labels),
    "firing": triggered_action.firing,
    "firings": triggered_action.all_firings(),
    "events": triggered_action.all_events(),
}

And discuss them in detail, and then we can talk about the shortcuts like flow_run and the tradeoffs of using them.

[rcash]

Documenting the jinja filters available as part of this would also be quite useful!