Revamp README for ReactiveUI.Avalonia with focused docs (#5)

* Revamp README for ReactiveUI.Avalonia with focused docs

Updated the README to focus on ReactiveUI.Avalonia, including new badges, a dedicated tutorial for Avalonia integration, simplified NuGet/package info, and improved support and contribution sections. Removed unrelated platform/package tables and team member listings to streamline documentation for Avalonia users.

* Update README.md

Author: ChrisPulman