[maui-labs docs] Update platform backend docs: new macOS template, --prerelease flag for all platform templates, macOS API fixes

[github-actions]

Closes #3311

Source PR

dotnet/maui-labs#251 — feat: Add macOS template, align platform READMEs, fix template packaging

Summary

This PR creates documentation for the experimental .NET MAUI platform backends (macOS AppKit, Linux GTK4, Windows WPF) in a new docs/developer-tools/platform-backends/ section, incorporating three user-facing changes from maui-labs PR #251:

1. New macOS dotnet new template — documents Microsoft.Maui.Platforms.MacOS.Templates and dotnet new maui-macos
2. --prerelease flag for all template installs — all three platform template install commands now include --prerelease
3. macOS namespace and API fix — uses correct using Microsoft.Maui.Platforms.MacOS.Platform; namespace in MauiMacOSApp.cs and includes using Microsoft.Maui.Platforms.MacOS.Essentials; + .AddMacOSEssentials() in MauiProgram.cs

Files changed

• docs/developer-tools/platform-backends/index.md (new) — Overview with template quick start commands for all three backends and a package reference table
• docs/developer-tools/platform-backends/macos.md (new) — macOS AppKit backend: prerequisites, packages, template quick start (Option 1), manual setup (Option 2) with corrected namespace and Essentials API
• docs/developer-tools/platform-backends/linux-gtk4.md (new) — Linux GTK4 backend: prerequisites, GTK4 install commands, packages, template quick start (Option 1), manual setup (Option 2) with --prerelease flag
• docs/developer-tools/platform-backends/windows-wpf.md (new) — Windows WPF backend: prerequisites, packages, template quick start (Option 1), manual setup (Option 2) with --prerelease flag
• docs/TOC.yml — Added "Platform backends" section under "Developer tools" with entries for all four new pages
• docs/developer-tools/index.md — Added "Platform backends" section summary, updated ms.date, added template packages to the packages table

Generated by Docs from Code — Create Draft PR for issue #3311 · ◷

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

More information about PoliCheck

Information: PoliCheck | Severity Guidance | Term
For any questions: Try searching the learn.microsoft.com contributor guides or post your question in the Learn support channel.

[learn-build-service-prod]

Learn Build status updates of commit b8ba54a:

💡 Validation status: suggestions

File	Status	Preview URL	Details
docs/developer-tools/platform-backends/linux-gtk4.md	💡Suggestion	View (>=net-maui-8.0)	Details
docs/developer-tools/index.md	✅Succeeded	View (>=net-maui-8.0)
docs/developer-tools/platform-backends/index.md	✅Succeeded	View (>=net-maui-8.0)
docs/developer-tools/platform-backends/macos.md	✅Succeeded	View (>=net-maui-8.0)
docs/developer-tools/platform-backends/windows-wpf.md	✅Succeeded	View (>=net-maui-8.0)
docs/TOC.yml	✅Succeeded	View (>=net-maui-8.0)

docs/developer-tools/platform-backends/linux-gtk4.md

• Line 0, Column 0: [Suggestion: filename-incomplete - See documentation] The filename contains an incomplete word, misspelled word, acronym, or abbreviation that is disallowed.

For more details, please refer to the build report.

Note: Your PR may contain errors or warnings or suggestions unrelated to the files you changed. This happens when external dependencies like GitHub alias, Microsoft alias, cross repo links are updated. Please use these instructions to resolve them.