Follow-up to #1023 / #1024 / #1025 - Xamarin references

**_Reason for opening this issue:_**

Follow-up to #1023 / #1024 / #1025.
That PR fixes the dead fabulous.dev links, but the surrounding text on both /guides/mobile-apps/ and /use/mobile-apps/ still describes a discontinued stack.

Xamarin.Forms has been retired (Microsoft moved to .NET MAUI), so "Option 1: ... using Xamarin" (/use/mobile-apps/) no longer applies.
Fabulous also moved off Xamarin.Forms — it now targets .NET MAUI and Avalonia (fabulous-dev/Fabulous).
The sample apps (Fabulous Weather, FabulousContacts) are old Xamarin.Forms-era examples.

**_Issue Location:_**
/use/mobile-apps/index.md
https://fsharp.org/use/mobile-apps/

**_Issue:_**
Xamarin.Forms has been retired by Microsoft in favor of .NET MAUI, so the former Xamarin-based workflow is no longer the recommended path.

**_Suggested fix:_**

### Option 1: Build Android and iOS Apps using F# and Fabulous

[Fabulous](https://github.com/fabulous-dev/Fabulous) is F#'s declarative, MVU-style UI framework and now targets .NET MAUI and Avalonia, letting you build apps for iOS, Android, macOS, Windows and Linux from a single F# codebase.

- [Fabulous on GitHub](https://github.com/fabulous-dev/Fabulous) — source, README and getting-started instructions

_// Links to sample apps to be discontinued_

*********************************************************************

**_Issue Location:_**
/guides/mobile-apps/index.md
https://fsharp.org/guides/mobile-apps/

**_Issue:_**
dtto

**_Suggested fix:_**

## Frameworks

* [Fabulous](https://github.com/fabulous-dev/Fabulous) - F# functional app development for Android, iOS, macOS, Windows and Linux, using the MVU (Model-View-Update) architecture on top of .NET MAUI and Avalonia.

Start with /use/mobile-apps/index.md and /guides/mobile-apps/index.md, focusing on the sections identified in the issue. Update the Xamarin.Forms and Fabulous descriptions to reflect .NET MAUI and Avalonia, remove the discontinued sample-app links, and verify both rendered pages no longer describe the retired stack.

Vom Indexierungsmodell aus dem Issue-Text verfasst.