added duo adoption metrics solution description

This change adds documentation for a new GitLab Duo Usage Metrics solution.

The documentation explains how organizations can track and measure their return on investment from GitLab's AI-powered Duo features by collecting detailed usage data that goes beyond standard dashboards.

The solution consists of three main components: a data collection tool that gathers Duo usage information through GitLab's API, an automated pipeline that runs periodically to collect and process this data, and a web dashboard that displays adoption trends and usage patterns. The documentation provides step-by-step setup instructions, including how to configure the necessary permissions and variables, schedule data collection, and deploy the analytics dashboard.

The guide covers both quick-start scenarios for users who only want Duo metrics and more advanced customization options for extending the system with additional metrics. It explains the technical architecture using a Python-based data collector and GraphQL API client, while emphasizing that the solution works specifically with GitLab's self-hosted and dedicated instances where direct API access is available.