Add minimal rclpy timer example demonstrating create_timer() usage

rclpy/timers/minimal_timer/README.md

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+# Minimal Timer Example
+
+This example demonstrates how to create a simple timer node in ROS 2 using rclpy.
+
+## Overview
+
+ROS 2 timers allow nodes to execute a callback function periodically.
+They are commonly used for tasks such as monitoring loops, periodic updates,
+or scheduled processing.
+
+This example creates a node that executes a callback every second using
+`create_timer()`.
+
+## Running the Example
+
+Run the node with:
+
+```bash
+python3 minimal_timer.py
+
+```
+
+You should see log messages printed once per second.
+
+## Example output
+
+Timer callback triggered
+Timer callback triggered
+Timer callback triggered
+
+## Key Concept
+
+`create_timer()` schedules a callback to be executed periodically.

rclpy/timers/minimal_timer/minimal_timer.py

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+import rclpy
+from rclpy.node import Node
+
+
+class MinimalTimer(Node):
+
+    def __init__(self):
+        super().__init__('minimal_timer')
+
+        timer_period = 1.0
+        self.timer = self.create_timer(timer_period, self.timer_callback)
+
+    def timer_callback(self):
+        self.get_logger().info('Timer callback triggered')
+
+
+def main(args=None):
+    rclpy.init(args=args)
+
+    minimal_timer = MinimalTimer()
+
+    rclpy.spin(minimal_timer)
+
+    minimal_timer.destroy_node()
+    rclpy.shutdown()
+
+
+if __name__ == '__main__':
+    main()