Add minimal_timer example for rclpy to demonstrate periodic execution using create_timer()

Description

The rclcpp examples repository contains minimal timer demonstrations (e.g., examples_rclcpp_minimal_timer), which help beginners understand how to execute periodic callbacks using ROS 2 timers.

However, the rclpy examples currently include:

• minimal_publisher
• minimal_subscriber
• pointcloud_publisher

but there is no minimal example demonstrating the use of create_timer() in Python.

Since timers are a fundamental mechanism in ROS 2 for running periodic tasks (such as sensor polling, monitoring loops, or periodic logging), a minimal Python example could help beginners understand this concept more easily.

Proposed Addition

Add a simple example under:

rclpy/topics/minimal_timer/

Possible structure:

rclpy/timer/minimal_timer/
 ├── minimal_timer.py
 └── README.md

The example could demonstrate:

• Creating a ROS 2 node
• Creating a timer using create_timer()
• Executing a periodic callback
• Logging output at fixed intervals

Motivation

This would:

• Improve parity between rclcpp and rclpy examples
• Help beginners understand periodic execution in ROS 2
• Provide a simple reference implementation for timers in Python

If this addition is considered useful, I would be happy to implement the example and submit a PR.

Compare the existing rclcpp minimal timer demonstrations and the current rclpy examples, especially minimal_publisher and minimal_subscriber. Then add the proposed rclpy/timer/minimal_timer example with minimal_timer.py and README.md, covering node creation, create_timer(), a periodic callback, and fixed-interval logging. Done means the example is runnable and provides the requested beginner reference.