docs: organize ecosystem projects by platform

[w3lld1]

Summary

• I grouped the ecosystem libraries by application platform so GTK and .NET MAUI integrations are easier to discover.
• I added the GirCore-backed SkiaSharp views and the experimental .NET MAUI Linux GTK4 backend, with links to their code, packages, and documentation.
• I separated FlatpakGenerator into a tooling section and corrected the stale “edit this page” link.

Validation

• git diff --check origin/main...HEAD
• Targeted Markdown heading/link validation against the exact commit
• Verified all 10 unique external links return HTTP 2xx

A full DocFX build was not run because a .NET SDK is not available in this environment; this is a documentation-only change.

Fixes #1568
Fixes #1569
Fixes #1570

• I agree that my contribution may be licensed either under MIT or any version of LGPL license.

[badcel]

Thank you.

Can you add a link to the Maui Nuget: https://www.nuget.org/packages/Microsoft.Maui.Platforms.Linux.Gtk4/

The correct repository link for skia sharp would be: https://github.com/mono/SkiaSharp/tree/main/source/SkiaSharp.Views/SkiaSharp.Views.Gtk4

The correct Nuget for skia sharp is: https://www.nuget.org/packages/SkiaSharp.Views.Gtk4/