[WinUI OSS] Create setup guide for new contributors

## Summary

Create or refresh a clear setup guide that helps new contributors move from a clean checkout to a successful WinUI build or validation step.

## Context

Open development depends on contributors being able to set up the repository without private knowledge. A good setup guide should describe prerequisites, clone/setup steps, expected commands, common failures, and the smallest useful validation path.

This work should be validated by maintainers or contributors following the guide from a clean environment so the documented path is realistic.

## Work to do

- Define the recommended new-contributor setup path from clean checkout to first useful validation.
- Document prerequisites, expected commands, and common troubleshooting steps.
- Link to existing public docs where they are authoritative instead of duplicating too much content.
- Validate the guide by walking through the steps and recording gaps as follow-up tasks.
- Coordinate with README, contribution docs, package restore, and build validation tasks.

## Definition of done

- New contributors have a clear public setup path.
- The guide has been validated against a clean or representative environment.
- Known setup gaps are linked to concrete follow-up issues.
- The guide contains no private package sources, private machine paths, or internal-only workarounds.

Start with the README and existing contribution docs, then trace the package restore and build validation tasks from a clean checkout. Walk through the recommended setup path in a clean or representative environment and record prerequisites, commands, common failures, and authoritative public links. Done means the guide supports first validation without private sources or paths, with remaining gaps linked to follow-up issues.

Written by the indexing model from the issue text.