Add documentation for how to use actors with the go-sdk

[greenie-msft]

Actors support is now available in the go-sdk. Let's add the necessary documentation on how to use actors with the sdk.

Content should be located in the daprdocs directory of the repo, which will appear here: https://docs.dapr.io/developing-applications/sdks/go/. It should look similar to how other sdks approached this: https://docs.dapr.io/developing-applications/sdks/python/python-actor/

[saranvasu]

Shall I pick this?

[msfussell]

@saranvasu - Are you still interested in creating any docs here?

[rclayton-godaddy]

@msfussell I'd be interested in writing the docs if I had an example to go by.

[rclayton-godaddy]

Ok - I'm a dummy. I see it in the examples/ folder.

[mikeee]

Addressed by #431