Add config for input devices

docs/docs/main/docs/configuration/input_device/index.md

@@ -8,6 +8,122 @@ All input devices are defined in the `[input_device]` table. Currently supported
 
 Please refer to the corresponding documentation for detailed configuration settings.
 
+## Processor Chain Configuration
+
+The `processor_chain` field in the `[input_device]` section allows you to specify the order in which processors handle events. This is useful when you have multiple processors and want to control their execution order.
+
+### Configuration
+
+```toml
+[input_device]
+processor_chain = [
+    "pmw3610_trackpoint_processor",  # Process trackpoint first
+    "joystick_thumbstick_processor",  # Then joystick
+    "scroll_processor",               # Then custom scroll processor
+    "battery_processor",              # Then battery updates
+]
+```
+
+### Complete Example
+
+Here's a full example showing how processor names are generated:
+
+**keyboard.toml:**
+```toml
+# PMW3610 sensor with custom name
+[[input_device.pmw3610]]
+name = "trackball"
+# ... other config
+
+# Joystick with custom name
+[[input_device.joystick]]
+name = "thumbstick"
+# ... other config
+
+# BLE battery monitoring
+[ble]
+enabled = true
+battery_adc_pin = "P0_04"
+
+# Specify processor order
+[input_device]
+processor_chain = [
+    "trackball_processor",           # PMW3610 (name="trackball")
+    "joystick_processor_thumbstick", # Joystick (name="thumbstick")
+    "scroll_processor",              # Custom processor
+    "battery_processor",             # Battery monitoring
+]
+```
+
+**src/main.rs:**
+```rust
+#[rmk::keyboard]
+mod my_keyboard {
+    // Custom processor - name matches function name
+    #[processor]
+    fn scroll_processor() -> ScrollWheelProcessor {
+        ScrollWheelProcessor::new(&keymap)
+    }
+}
+```
+
+Generated processor names:
+- `trackball_processor` ← from `name = "trackball"`
+- `joystick_processor_thumbstick` ← from `name = "thumbstick"`
+- `scroll_processor` ← from function name
+- `battery_processor` ← auto-generated for battery
+
+### Processor Names
+
+Processor names are automatically generated based on your configuration. Here's how to determine available processor names:
+
+#### Built-in Processors
+
+RMK automatically creates processors for configured input devices:
+
+| Device Type | Naming Rule | Example Configuration | Generated Name |
+|-------------|-------------|----------------------|----------------|
+| **PMW3610** | `{name}_processor` | `[[input_device.pmw3610]]`<br>`name = "trackball"` | `trackball_processor` |
+| **PMW3610** (no name) | `pmw3610_{idx}_processor` | `[[input_device.pmw3610]]`<br>(first device, no name) | `pmw3610_0_processor` |
+| **Joystick** | `joystick_processor_{name}` | `[[input_device.joystick]]`<br>`name = "left_stick"` | `joystick_processor_left_stick` |
+| **Battery** | `battery_processor` | BLE battery ADC enabled | `battery_processor` |
+
+::: tip How to find your processor names
+If you use an invalid processor name in `processor_chain`, the compiler will show an error listing all available processors. This is the easiest way to discover processor names!
+:::
+
+#### Custom Processors
+
+Custom processors use the **function name** from your Rust code:
+
+```rust
+#[processor]
+fn scroll_processor() -> ScrollWheelProcessor {  // Name: "scroll_processor"
+    ScrollWheelProcessor::new(&keymap)
+}
+
+#[processor]
+fn my_custom_filter() -> MyFilter {  // Name: "my_custom_filter"
+    MyFilter::new()
+}
+```
+
+### Default Order
+
+If `processor_chain` is not specified, processors run in this order:
+1. All built-in processors (in TOML definition order)
+2. All custom processors (in Rust code definition order)
+
+### How It Works
+
+Each processor in the chain receives events and can either:
+- **Continue**: Pass the event (possibly modified) to the next processor
+- **Stop**: Event is fully handled, chain stops
+
+This allows you to create sophisticated event processing pipelines with filtering, transformation, and routing logic.
+
+For more details on creating custom processors, see the [Input Device Features](../../features/input_device.md#custom-processors) documentation.
+
 ## Configuring Multiple Input Devices
 
 You can define and configure any number of input devices. To add multiple instances of a device, simply repeat the device type sub-table in your configuration: