Harmonic: Add information to reproduce demo in ros_gz_project_template

## Desired behavior
The [ros_gz_project_template docs](https://gazebosim.org/docs/harmonic/ros_gz_project_template_guide) explain how to build and install, but are missing the instructions how to move the diffdrive robot in [this section](https://gazebosim.org/docs/harmonic/ros2_integration#visualize-in-rviz)/

## Alternatives considered

I've spent two hours trying various methods, nothing seems to move the vehicle.

With this command, the vehicle moves in RVIZ, but not in gazebo.
```bash
ros2 topic pub /diff_drive/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 0.5}, angular: {z: 0.5}}"
```

## Implementation suggestion

Add a small section to the end of the `ros_gz_project_template_guide` that explains the commands in terminal or through the gazebo UI to move the vehicle.

## Additional context

Gazebo Version: Harmonic 8.2.0
OS: Ubuntu 22
Installation method: Binaries
Discord chat: https://discord.com/channels/1077825543698927656/1080763334384697430/1230559471659388991

Bag is attached.
[rosbag2_2024_04_18-15_54_56.zip](https://github.com/gazebosim/docs/files/15030963/rosbag2_2024_04_18-15_54_56.zip)

Inspection shows the odometry is updating, so why doesn't gazebo UI show it moving?

Start with the ros_gz_project_template_guide and the linked ROS 2 integration section, then reproduce the diff-drive example on Gazebo Harmonic 8.2.0 using the documented topic command and available Gazebo UI controls. Done means the guide includes a small section with commands or UI steps that reliably moves the robot in Gazebo.

Written by the indexing model from the issue text.