Add information about how actor state relation to durable state stores

[elena-kolevska]

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 #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