Add minimal rclpy timer example demonstrating create_timer() usage

[yadnyeshwar]

This PR adds a minimal example demonstrating how to use
create_timer() in rclpy.

While the minimal publisher example already uses a timer internally,
this example focuses specifically on timer usage without publishing.

This provides a simple reference for beginners learning
periodic execution in ROS 2.

Closes #436

[alsora]

Adding the python file is not sufficient.

you should create a package so that the script can be "built" and installed, and then run via ros2 run

[fujitatomoya]

@yadnyeshwar this is requested changes, friendly ping.

[yadnyeshwar]

@yadnyeshwar this is requested changes, friendly ping.

Thank you for the reminder and apologies for the delayed response. I was away for a short period due to personal reasons.

I have now addressed the requested changes by converting the example into an installable ROS 2 Python package so that it can be built and executed via ros2 run. I am currently verifying the CI results and will follow up with any additional fixes if needed.

Please let me know if there are any other adjustments you would like me to make.

[alsora]

this still requires work.

• there are two identical copies of the script (one under timers and one under topics)
• the new files (setup, package, etc) were added to the topics folder rather than to the timers folder, where they should be.