[WinUI OSS] Update WinUI documentation for the open-source contribution path

## Summary

Review WinUI documentation so it consistently points developers to the public repository, explains the open-source contribution path, and helps new developers get started.

## Context

WinUI documentation should make it clear that development happens in the open and that contributors can use the public GitHub repository for source, issues, and pull requests. Getting-started content should also reduce friction by linking to current setup guidance and modern project creation commands where appropriate.

This issue should focus on public documentation quality. Avoid exposing internal ownership notes, unpublished roadmap details, or private planning references.

## Work to do

- Identify public WinUI documentation pages that should link to the GitHub repository or contribution guidance.
- Update getting-started content where current setup or project creation steps are missing or stale.
- Ensure docs explain where to file issues, where to find source, and how to start contributing.
- Coordinate with repository README/contribution-doc updates so docs and repo guidance agree.
- Track any documentation changes that require separate review or ownership.

## Definition of done

- Key WinUI docs point to the public repository and contribution path.
- Getting-started guidance is clearer and aligned with current tooling.
- Stale links or outdated contribution references are removed or tracked.
- Public documentation avoids private roadmap, ownership, or internal planning details.

Start by reviewing the public WinUI documentation pages, repository README, and contribution guidance. Compare repository, issue, pull request, setup, and project-creation links with the current tooling, then update stale or missing guidance. Done means the key docs consistently explain the public contribution path and exclude private planning details.

Written by the indexing model from the issue text.