docs(cloud-run): Clarify multi-agent project structure for Cloud Run deployment

[triangle-z]

• This update clarifies how to define and deploy multiple agents using Cloud Run.
• It specifies that each agent should be placed in its own folder at the root level, with each folder containing a root_agent.
• It also shows an example structure, in complement of the one in Project Structure.

Here's how it looks :

[lavinigam-gcp]

LGTM, merging