Core API example showing multiple agents concurrently

[gziz]

Why are these changes needed?

This example demonstrates multiple agents running concurrently, as mentioned in #4427.

Note: Some additional updates are still required, such as:

• Linking this notebook to display it in the documentation.
• Possibly adding more detailed explanations for each scenario.

I wanted to share this draft to confirm if these are the types of examples you were looking for, @ekzhu. Thank you!

Related issue number

#4427

Checks

• [ ] I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• [ ] I've added tests (if relevant) corresponding to the changes introduced in this PR.
• [ ] I've made sure all auto checks have passed.