[WinUI OSS] Refresh WinAppSDK CLI documentation

## Summary

Refresh WinAppSDK CLI documentation so the setup and project creation path is current, clear, and aligned with WinUI contribution guidance.

## Context

CLI-based project setup is one of the fastest ways for developers to try WinUI and related Windows App SDK experiences. If CLI docs are stale, contributors may hit setup failures before they can evaluate or contribute to the project.

This task should align public docs with current commands, supported templates, and repository guidance without referencing private planning or internal documentation.

## Work to do

- Review WinAppSDK CLI documentation for stale commands, outdated screenshots, unclear prerequisites, or missing links.
- Validate the recommended commands against the current public setup path where possible.
- Link to WinUI repository guidance when the docs discuss contributing, samples, or source.
- Track documentation gaps that require owners outside the WinUI repository.
- Avoid private documentation links, internal rollout plans, or unpublished tooling details.

## Definition of done

- CLI documentation reflects current public guidance.
- Developers can follow the docs without private context.
- Cross-links to WinUI setup and contribution guidance are correct where appropriate.
- Remaining documentation gaps are tracked with clear owners or follow-up issues.

Locate the public WinAppSDK CLI documentation covering setup and project creation, then compare its commands, templates, prerequisites, screenshots, and links with the current public setup path. Validate the recommended flow where possible, add correct WinUI setup and contribution cross-links, and record remaining gaps with clear owners or follow-up issues.

Written by the indexing model from the issue text.