Add AndroidX installation guide and update docs (#929)

* Add AndroidX installation guide and update docs

Introduces a comprehensive AndroidX (Xamarin.Android) installation and usage guide. Updates table of contents to reference AndroidX instead of legacy Xamarin Android, and revises the Xamarin Android guidelines to focus on AndroidX. Removes references to deprecated Xamarin platforms from documentation navigation.

* Update Android framework requirements in docs

Raised the minimum required Android version to 14.0 (API Level 34) and updated the target framework to net9.0-android in the installation guide for AndroidX.

Author: ChrisPulman

reactiveui/docs/getting-started/installation/toc.yml

@@ -14,11 +14,5 @@
   href: windows-presentation-foundation.md
 - name: Windows UI
   href: winui.md
-- name: Xamarin Android (legacy)
-  href: xamarin-android.md
-- name: Xamarin Forms (legacy)
-  href: xamarin-forms.md
-- name: Xamarin iOS (legacy)
-  href: xamarin-ios.md
-- name: Xamarin Mac (legacy)
-  href: xamarin-mac.md
+- name: AndroidX
+  href: androidx.md