[WinUI OSS] Create GitHub roadmap template

## Summary

Create a public roadmap template for the WinUI repository so plans and priorities can be communicated consistently.

## Context

A roadmap template can help maintainers share direction without overcommitting to private schedules or internal planning details. The template should make it easy to communicate themes, work status, dependencies, and known caveats in a way that is useful to contributors.

This issue should focus on the template structure and publishing workflow, not on filling in private roadmap content.

## Work to do

- Identify what roadmap information should be public: themes, status, target areas, dependencies, and contribution opportunities.
- Define fields or sections that avoid private dates, private commitments, and internal planning terminology.
- Decide where the roadmap should live and how it should link to GitHub issues or project views.
- Draft template language that maintainers can reuse safely.
- Coordinate with documentation and project-board tracking so roadmap items point to useful public work items.

## Definition of done

- A public-safe roadmap template is drafted or added to the repository.
- The template explains how roadmap entries should connect to GitHub issues or project tracking.
- Maintainers understand what should and should not be included.
- The template avoids private roadmap commitments, internal milestones, and non-public dependencies.

No specific file or test is named. Start by reviewing the repository’s documentation and GitHub project-board tracking practices, then identify where the reusable roadmap template and publishing guidance should live. Done means the public-safe sections, linking workflow, and exclusions for private commitments are documented for maintainers.

Written by the indexing model from the issue text.