Document actor events better

[fnesveda]

There are some events being sent to the actor runs over websocket about what is happening with the actor run (run is aborting, migrating etc.). We have them described a bit in the SDK docs, but the documentation is outdated. We should update the docs and move them to the general actor docs, because the events are not specific just to Javascript actors, any actor can use them.

[mtrunkat]

Good point, I added a page in the platform docs - https://docs.apify.com/platform/actors/development/programming-interface/system-events

So assigning t-tooling to update SDK docs and link this from there to have it centralized at a single place.