[Docs]: Add winapp CLI alongside existing Microsoft Learn Windows app workflows

### What part of the documentation needs improvement?

Official Microsoft Learn guidance for WinUI, Windows App SDK, packaging, app identity, signing, and related workflows should be reviewed for appropriate integration with the `winapp` CLI.

Initial targets include:

- WinUI getting-started tutorials
- The existing Windows App Development CLI documentation
- Packaging and MSIX guidance
- Package identity and sparse packaging guidance
- Certificate generation, signing, and deployment guidance

### Describe the change you'd like

Audit relevant Microsoft Learn documentation and present `winapp` workflows side by side with existing Visual Studio, MSBuild, and .NET CLI instructions where appropriate.

The updates should:

- Identify Learn pages where a `winapp` path would benefit users.
- Add tested, end-to-end commands and expected outcomes.
- Preserve existing workflows while clearly explaining when each option is useful.
- Cross-link relevant pages to the central `winapp` CLI documentation.
- Keep terminology consistent across WinUI, packaging, identity, certificates, signing, and deployment.
- Clearly distinguish full MSIX packaging, sparse packaging, and unpackaged scenarios.
- Coordinate changes with the owners of the affected Microsoft Learn repositories.
- Use a preview branch or staged review so the complete experience can be tested before publication.

### Additional context

The current Learn documentation mentions the CLI in isolation, but broader Windows app guidance does not consistently surface it as an available option. Now that `winapp` supports project creation, `.csproj` generation, packaging, and sparse identity workflows, this is a good point to add a cohesive CLI path alongside the established Visual Studio and .NET experiences.