Docs: Add Customer Service Collaboration Use Case

[jeffwhelpley]

This pull request adds a detailed use case for "Customer Service Collaboration" to the use_case.html document, filling in the previously empty section.

The new section describes a common scenario where a customer's Personal Agent interacts with a company's Service Agent (e.g., "ServiceAir") to reschedule a delayed flight. It details a 4-step process:

1. Requirements Gathering: The Personal Agent collects reservation details and new flight parameters.
2. Authentication and Flight Lookup: The Personal Agent authenticates with the ServiceAir Agent and validates the booking.
3. Rescheduling Collaboration: The agents exchange information on available flights, supporting both autonomous selection (based on user preferences) and a human-in-the-loop for final approval.
4. Rebooking Transaction: The agents complete the rebooking and confirmation.

This use case helps illustrate the practical, real-world value of agent-to-agent protocols and connects directly to several key requirements, including Agent Discovery, Agent Identity Authentication, Agent Transactions, and Privacy Protection.

Future Work & Contribution

This initial use case focuses on a transactional scenario requiring an official company agent. I recognize there are many other customer service scenarios, such as information-gathering, where a Personal Agent might collaborate with any knowledgeable third-party agent.

I have professional experience in this domain (my startup, GetHuman, specializes in customer service) and would be happy to help flesh out more detailed agent-to-agent customer service interactions. For this PR, I wanted to start with one simple, concrete example. I plan to attend the next bi-weekly meeting and can discuss then how else I might be able to contribute.