[WinUI OSS] Refresh contribution and repository landing docs

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## Summary

Refresh the repository landing and contribution documentation so new contributors can understand how WinUI is developed in the open and how to get started.

## Context

The repository README and contribution guidance are the first places many contributors will look before filing issues, building locally, or opening pull requests. These files should describe the current GitHub-based workflow, expected contribution path, review expectations, and links to build/setup documentation.

The work should use public examples and public repository conventions. If reviewing other open-source Microsoft repositories for inspiration, capture the resulting guidance generically instead of copying private team discussions or internal process notes.

## Work to do

- Review the current README and contribution guidance for stale setup, build, and workflow instructions.
- Clarify how contributors should file issues, choose labels or templates, open pull requests, and respond to review feedback.
- Add links to current public build/setup docs and remove references that are no longer accurate.
- Document expectations for AI-assisted or generated contributions in public-safe, contributor-friendly language if that policy is ready to share.
- Coordinate with setup-guide, label-cleanup, and bot/command tasks so the docs point to the right contributor workflow.

## Definition of done

- Repository landing and contribution docs reflect the current open-development workflow.
- New contributors have clear next steps for setup, issue filing, and pull requests.
- Outdated or confusing guidance has been removed or replaced.
- The public docs avoid private policy discussions, private process links, and internal-only escalation paths.

Start by reviewing the repository README and contribution guidance, then compare their setup, build, issue, label, template, pull request, and review instructions with the current public workflow. Check the public build/setup documentation and coordinate with the setup-guide, label-cleanup, and bot/command tasks. Done means contributors have clear current next steps and no private or outdated guidance remains.

Written by the indexing model from the issue text.