SDK for JS docs is missing proper explanation of Actor lifecycle methods (`Actor.init` & `Actor.exit`)

[jbartadev]

The only proper explanation is in Python docs here - it should be described probably on the main level somewhere.

This comes from the discussion we had in this PR review

[mtrunkat]

It's described in the platform docs (https://docs.apify.com/platform/actors/development/programming-interface/basic-commands) but not properly described in the SDK for JS docs. So assigning @B4nan 's team and adding SDK to title