Update ADK doc according to issue #1029 - 1

[adk-bot]

This PR refactors the agent-team.md tutorial to use the new TransferToAgentTool as requested in issue #1029, part 1.

This change involves:

• Introducing the TransferToAgentTool in Step 3 of the tutorial as the best practice for reliable delegation.
• Updating the root agent's definition in Steps 3, 4, 5, and 6 to consistently use this explicit and constrained tool.
• Modifying the agent's instructions and the explanatory text to align with this more robust delegation mechanism.