[WinUI OSS] Add WinUI Gallery usage telemetry

## Summary

Add public-safe usage telemetry planning for WinUI Gallery so maintainers can understand feature usage and prioritize improvements.

## Context

WinUI Gallery is an important sample and discovery surface for WinUI controls and patterns. Maintainers need enough usage signal to understand which samples are valuable, where users encounter friction, and which areas may need better documentation or examples.

This issue should focus on telemetry goals, privacy expectations, and implementation planning. Do not include private telemetry endpoints, keys, dashboards, tenant details, or internal reporting links in the public issue.

## Work to do

- Define the public-safe questions WinUI Gallery telemetry should answer.
- Identify events or usage signals that are useful without collecting unnecessary personal data.
- Confirm privacy, consent, and documentation expectations before implementation.
- Track implementation work for event wiring, validation, and documentation as separate follow-ups if needed.
- Ensure public discussion avoids private endpoints, dashboards, credentials, or internal reporting details.

## Definition of done

- Telemetry goals and privacy expectations are documented.
- Implementation follow-ups are scoped clearly enough for engineers to start.
- Maintainers understand how usage signal will inform WinUI Gallery improvements.
- Public issue content avoids private telemetry infrastructure and sensitive data details.

Start by reviewing the issue's telemetry goals, privacy, consent, and public-disclosure requirements. Document the public-safe questions and usage signals WinUI Gallery needs, then define implementation follow-ups for event wiring, validation, and documentation. Done means the goals and privacy expectations are documented and the follow-ups are clear enough for engineers to begin.

Written by the indexing model from the issue text.