Finish Actors Docs

[greenie-msft]

At the moment, Actors is unfinished in the Java SDK docs: https://docs.dapr.io/developing-applications/sdks/java/#actors

The only example it shows is an actor interface for "DemoActor".

As a Java developer interested in Actors via the SDK, I expect to see an example of DemoActor implementation and an example of a client calling the methods on the DemoActor.

[artursouza]

See README as an example: https://github.com/dapr/java-sdk/tree/master/examples/src/main/java/io/dapr/examples/actors