Revise getting started guide with RxAppBuilder and ecosystem (#920)

Expanded the getting started documentation to highlight RxAppBuilder as the recommended initialization method, added a modern code sample, and reorganized key features and ecosystem libraries. Updated MVVM explanation, provided more resources, and clarified next steps for new users.

Author: ChrisPulman

reactiveui/docs/getting-started/index.md

@@ -6,31 +6,79 @@ See our <a href="~/docs/handbook/index.md">Handbook</a> for the ReactiveUI docum
 
 ## Getting Started
 
-To get started visit our <a href="~/docs/getting-started/installation/index.md">Installation</a> page.
+To get started visit our <a href="~/docs/getting-started/installation/index.md">Installation</a> page to install the appropriate NuGet packages for your platform.
 
-The [Compelling Example](compelling-example.md) walks through creating a more complete application, demonstrating the power of ReactiveUI and Reactive Extensions.
+### Modern ReactiveUI with RxAppBuilder (Recommended)
 
-## Why MVVM?
+**RxAppBuilder** is the recommended way to initialize and configure ReactiveUI applications (introduced in v21.0.1). It provides a fluent API for setting up dependency injection, view/view model registration, and platform-specific services:
+
+```csharp
+var app = RxAppBuilder.CreateReactiveUIBuilder()
+    .WithWpf() // Or WithMaui(), WithBlazor(), WithWinUI(), etc.
+    .WithViewsFromAssembly(typeof(App).Assembly)
+    .WithRegistration(locator =>
+    {
+        // Register your services
+        locator.RegisterLazySingleton<IScreen>(() => new MainViewModel());
+    })
+    .BuildApp();
+```
 
-The Model-View-ViewModel (MVVM) pattern helps create more portable and maintainable codebases for cross-platform .Net applications. It increases the amount of code that can be shared between different platforms (iOS, Android, etc.) and makes testing easier.
+Learn more about RxAppBuilder in the <a href="~/docs/handbook/rxappbuilder.md">RxAppBuilder Guide</a>.
+
+### Key ReactiveUI Features
 
 ReactiveUI makes it easy to combine the MVVM pattern with Reactive Programming by providing features such as:
 
-- [WhenAnyValue](~/docs/handbook/when-any.md)
-- [ReactiveCommand](~/docs/handbook/commands/index.md)
-- [ObservableAsPropertyHelper](~/docs/handbook/observable-as-property-helper.md)
-- [WhenActivated](~/docs/handbook/when-activated.md)
-- [Data Binding](~/docs/handbook/data-binding/index.md)
+- **[RxAppBuilder](~/docs/handbook/rxappbuilder.md)** - Modern application initialization and dependency injection
+- **[ReactiveUI.SourceGenerators](https://github.com/reactiveui/ReactiveUI.SourceGenerators)** - Compile-time code generation for reactive properties and commands
+- **[WhenAnyValue](~/docs/handbook/when-any.md)** - Observe property changes reactively
+- **[ReactiveCommand](~/docs/handbook/commands/index.md)** - Asynchronous, composable command execution
+- **[ObservableAsPropertyHelper](~/docs/handbook/observable-as-property-helper.md)** - Transform observables into read-only properties
+- **[WhenActivated](~/docs/handbook/when-activated.md)** - Manage subscriptions and prevent memory leaks
+- **[Data Binding](~/docs/handbook/data-binding/index.md)** - Type-safe, reactive data binding
+- **[User Input Validation](~/docs/handbook/user-input-validation.md)** - Declarative validation with ReactiveUI.Validation
+
+The [Compelling Example](compelling-example.md) walks through creating a more complete application, demonstrating the power of ReactiveUI and Reactive Extensions.
+
+## Why MVVM?
+
+The Model-View-ViewModel (MVVM) pattern helps create more portable and maintainable codebases for cross-platform .NET applications. It increases the amount of code that can be shared between different platforms (Windows, iOS, Android, Web, etc.) and makes testing easier.
 
 <img src="~/images/mvvm.png" width="500" alt="mvvm">
 
-## Explore ReactiveUI
+## Explore the ReactiveUI Ecosystem
+
+ReactiveUI is much more than just a MVVM helper. Take a look at the following projects to get started exploring what is available:
+
+### Core Libraries
+
+- **[DynamicData](https://github.com/reactivemarbles/DynamicData)** - Reactive collections based on Rx.NET
+- **[ObservableEvents](https://github.com/reactivemarbles/ObservableEvents)** - Generate observables from .NET events
+- **[Splat](https://github.com/reactiveui/Splat)** - Cross-platform dependency injection and logging
+- **[Akavache](~/docs/handbook/akavache/toc.yml)** - Asynchronous key-value store with SQLite persistence
+- **[ReactiveUI.Validation](~/docs/handbook/user-input-validation.md)** - Reactive validation for user input
+
+### Platform Extensions
+
+- **[ReactiveUI.SourceGenerators](https://github.com/reactiveui/ReactiveUI.SourceGenerators)** - C# source generators for ReactiveUI
+- **[Maui.Plugins.Popup](https://github.com/reactiveui/Maui.Plugins.Popup)** - MAUI popup plugin with ReactiveUI support
+- **[Sextant](https://github.com/reactiveui/Sextant)** - Navigation library for Xamarin.Forms using ReactiveUI
+
+### Additional Tools
+
+- **[Fusillade](https://github.com/reactiveui/Fusillade)** - HTTP request prioritization and rate limiting
+- **[Punchclock](https://github.com/reactiveui/punchclock)** - Asynchronous work queue with prioritization
+
+### Resources
+
+- **[Samples](~/docs/resources/samples.md)** - Open source applications built with ReactiveUI
+- **[Blog](~/articles/2020-07-16-article-on-elevated-values.md)** - Release notes and announcements
+- **[Videos and Presentations](~/docs/resources/videos.md)** - Videos and presentations about ReactiveUI
 
-ReactiveUI is much more than just a MVVM helper. Take a look at the following projects to get started exploring what is available
+## Next Steps
 
-- [DynamicData](https://github.com/reactivemarbles/DynamicData) - Reactive collections based on reactive extensions
-- [ObservableEvents](https://github.com/reactivemarbles/ObservableEvents) - Build Observables from Events
-- [Sextant](https://github.com/reactiveui/Sextant)- Navigation library for Xamarin.Forms using ReactiveUI
-- [Samples](~/docs/resources/samples.md) - Open Source applications built with ReactiveUI
-- [Blog](~/articles/2020-07-16-article-on-elevated-values.md) - Release Notes and Announcements
-- [Videos and Presentations](~/docs/resources/videos.md) - Videos and Presentations
+1. **[Install ReactiveUI](~/docs/getting-started/installation/index.md)** for your platform
+2. **Follow the [Compelling Example](compelling-example.md)** to build your first reactive app
+3. **Read the [Handbook](~/docs/handbook/index.md)** to learn about advanced features
+4. **Join the Community** on [Slack](https://join.slack.com/t/reactivex/shared_invite/zt-lt48skpz-G5WDYOAuzA80_MByZrLT0g)