[WinUI OSS] Document WinUI API review process

Summary

Document the public-facing API review process for WinUI changes so contributors and maintainers understand how API proposals move from idea to implementation.

Context

API changes often need more structure than ordinary code changes. Contributors should know when an API review is required, what information to include, where proposals should be discussed, and how review outcomes are reflected in issues or pull requests.

This task should document the public process at a high level without exposing private meeting notes, internal approval chains, or non-public design discussions.

Work to do

• Identify the steps a contributor should follow when proposing a WinUI API change.
• Document required proposal information, review expectations, and how decisions are communicated.
• Link to public templates, examples, or guidance if available.
• Clarify how API-review work connects to issues, pull requests, labels, and project tracking.
• Avoid private meeting links, internal distribution lists, unpublished decisions, and internal escalation paths.

Definition of done

• Contributors have clear public guidance for API review.
• Maintainers have a consistent process to point contributors to.
• API-review-related labels, templates, or issue fields are identified if needed.
• Public documentation avoids private approval details and internal discussion links.