docs: Add Rust examples including JSON handling and Slint UI framework

Summary:
- Added two new Rust example applications to the documentation:
  - `hello` example showcasing JSON serialization/deserialization, async runtime usage, and C interoperability in a NuttX environment
  - `slint` example demonstrating the integration of the Slint UI framework with NuttX's framebuffer and touchscreen devices for creating GUI applications in Rust
- Each example includes detailed explanations of key features and how they are implemented using Rust's modern language capabilities

Impact:
- Enhances the documentation for Rust development within NuttX, providing clear and practical examples for developers
- Facilitates easier adoption of Rust for new projects by demonstrating how to leverage its features while maintaining compatibility with existing NuttX infrastructure
- No changes to the core NuttX system; the additions are entirely within the documentation and examples directory

Signed-off-by: Huang Qi <huangqi3@xiaomi.com>

Documentation/applications/examples/rust/hello/index.rst

@@ -0,0 +1,30 @@
+==================================
+`hello` Example in Rust
+==================================
+
+This example demonstrates how to use Rust's powerful features in a NuttX environment, including:
+
+- **JSON Serialization/Deserialization**: Using the popular `serde` and `serde_json` crates to work with JSON data
+- **Async Runtime**: Demonstrates basic usage of the `tokio` async runtime
+- **C Interoperability**: Shows how to expose Rust functions to be called from C code
+
+Key Features
+------------
+
+1. JSON Handling
+   - Defines a `Person` struct with `Serialize` and `Deserialize` traits
+   - Serializes Rust structs to JSON strings
+   - Deserializes JSON strings into Rust structs
+   - Demonstrates pretty-printing JSON
+
+2. Async Runtime
+   - Initializes a single-threaded `tokio` runtime
+   - Runs a simple async task that prints a message
+
+3. C Interop
+   - Exports `hello_rust_cargo_main` function with `#[no_mangle]` for C calling
+   - Uses `extern "C"` to define the C ABI
+
+The example shows how Rust's modern features can be used in embedded systems while maintaining compatibility with C-based systems.
+
+This example serves as a foundation for building more complex Rust applications in NuttX that need to handle JSON data and async operations.

Documentation/applications/examples/rust/slint/index.rst

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+==================================
+`slint` UI Framework Example
+==================================
+
+.. image:: slint.jpg
+   :alt: Slint UI Example Screenshot
+   :align: center
+
+This example demonstrates how to use the Slint UI framework in a NuttX environment with Rust. It shows how to create a simple GUI application that:
+
+- Displays a counter that increments every second
+- Uses NuttX's framebuffer device for rendering
+- Handles touchscreen input
+- Implements a software renderer for the Slint UI
+
+Key Features
+------------
+
+1. **Framebuffer Integration**
+   - Opens and configures the NuttX framebuffer device (/dev/fb0)
+   - Supports RGB565 pixel format
+   - Implements a custom line buffer provider for efficient rendering
+
+2. **Touchscreen Input**
+   - Opens and reads from the touchscreen device (/dev/input0)
+   - Handles touch press, move, and release events
+   - Maps touch coordinates to UI elements
+
+3. **Slint UI Framework**
+   - Creates a simple window with a text element showing a counter
+   - Uses Slint's software renderer backend
+   - Implements a custom NuttX platform adapter
+   - Handles window events and animations
+
+4. **Rust Integration**
+   - Uses nuttx crate to interface with NuttX devices
+   - Implements safe Rust abstractions for framebuffer and touchscreen
+   - Demonstrates Rust's memory safety features with unsafe blocks
+
+The example shows how modern Rust UI frameworks can be used in embedded systems while maintaining compatibility with NuttX's device model.
+
+This serves as a foundation for building more complex Rust GUI applications in NuttX that need to handle graphical output and touch input.