[Actors] Namespaced actors

[hhunter-ms]

What content needs to be created or modified?
Add documentation covering support for cross-namespace actor invocation

Where should the new material be placed?
Add a couple of paragraphs to existing actors documentation, like the Features section of the overview

The associated pull request from dapr/dapr, dapr/components-contrib, or other Dapr code repos
dapr/dapr#3786
dapr/dapr#7745

[elena-kolevska]

@hhunter-ms can we please change the name to "Namespaced actors"?

[hhunter-ms]

@hhunter-ms can we please change the name to "Namespaced actors"?

@elena-kolevska Done! I was just looking at this docs issue and the code PR. Will you be working on docs for this? I'm happy to provide support if necessary.