Add information about how actor state relation to durable state stores

**What content needs to be created or modified?**
We need to explain how actor state is saved, is it the user's responsibility or done automatically. I'm aware there is some overlap with https://github.com/dapr/docs/issues/4268, but this would be just a sentence or two on the overview page, not a deep dive.

**Where should the new material be placed?**
https://v1-15.docs.dapr.io/developing-applications/building-blocks/actors/actors-overview/#state