[WinUI OSS] Clean up GitHub wiki content

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## Summary

Review the GitHub wiki and clean up stale or duplicated content so contributors know which documentation source to trust.

## Context

Wiki pages can become stale when product documentation, repository docs, and Microsoft Learn content evolve independently. Contributors need a clear path to current setup, build, contribution, and troubleshooting guidance.

This task should decide which wiki pages remain useful, which should link to maintained documentation, and which should be removed or archived.

## Work to do

- Inventory wiki pages that relate to setup, builds, contribution, triage, samples, or project status.
- Identify stale, duplicated, or misleading content.
- Replace stale wiki content with links to maintained public docs where appropriate.
- Preserve useful context in public-safe language if a page is still valuable.
- Avoid private planning notes, internal-only process references, or non-public links.

## Definition of done

- Stale wiki content has been removed, updated, or redirected.
- Contributors can distinguish current guidance from historical notes.
- Maintainers have a clearer documentation ownership model.
- Public wiki content avoids internal process details and private links.

Start by inventorying the GitHub wiki pages covering setup, builds, contribution, triage, samples, and project status. Compare each page with the maintained repository documentation and Microsoft Learn content, then remove, update, archive, or redirect stale material. Done means contributors can identify current guidance and no public page contains private links or internal-only process details.

Written by the indexing model from the issue text.