Update Readme to include Copilot Studio, rather than just Azure Bot Service

[cleemullins]

Is your feature request related to a problem?

The current readme doesn't include any references to WebChat being the primary mechanism users interact with Agents based on Copilot Studio.

Describe the suggestion or request in detail

Webchat is used throughout Copilot Studio, and is a key portion of the overall experience.

Update the Readme to include details about how WebChat is used, and how the customization of WebChat is applicable to Agents created with Copilot Studio

Describe alternatives you have considered

No response

Additional context

No response