docs: add Google Chat ADK Agent Add-on to community showcase

[eliasecchig]

Summary

• Add showcase entry for Google Workspace tutorial on building a Google Chat app with ADK AI agent

Details

Adds the Google Chat ADK Agent Add-on showcase as requested in #658:

• Tutorial: Build a Google Chat app with an ADK AI agent
• Repository: googleworkspace/add-ons-samples

The tutorial demonstrates:

• Deploying the LLM Auditor ADK sample via Agent Starter Pack from GCP console
• Building a Google Chat app that interfaces with Vertex AI Agent Engine
• AI-powered fact-checking (e.g., users send "The Eiffel Tower was completed in 1900" and get corrections with sources)

Closes #658

[gemini-code-assist[bot]]

Summary of Changes

Hello @eliasecchig, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the project's documentation by integrating a new entry into the community showcase. The added content features a Google Chat ADK Agent Add-on, providing developers with a practical example of building AI-powered applications within Google Workspace. It includes essential resources like a tutorial and repository, detailing how to deploy and leverage advanced features such as multi-agent systems for fact-checking, thereby enriching the available learning materials for the community.

Highlights

• New Showcase Entry: Added a new entry to the community-showcase.md documentation.
• Google Chat ADK Agent Add-on: The entry highlights a Google Chat application built with an ADK AI agent.
• Resource Links: Includes direct links to an official tutorial and the corresponding GitHub repository for the add-on.
• Detailed Description: Provides a comprehensive description of the add-on's functionality, deployment methods (Agent Starter Pack, Cloud Shell), and key features such as quick setup, custom UI integration, and the LLM Auditor multi-agent pattern for fact-checking.
• Technology Stack: Lists the core technologies involved, including ADK, Vertex AI Agent Engine, Google Workspace Add-ons, Apps Script, and Google Chat.

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[jeremylongshore]

Is this related to Dialogflow CX?