Add a namespaced example (backport #457)

[mergify]

Similar to ros-controls/gazebo_ros2_control#181

But I haven't found a way to pass the namespace to ros_gz_sim create to automatically overwrite the namespace of the plugin, as it is possible with '-robot_namespace', 'r1' of gazebo_ros spawn_entity.py

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This is an automatic backport of pull request #457 done by Mergify.

[mergify]

Cherry-pick of a7d5a40 has failed:

On branch mergify/bp/humble/pr-457
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Unmerged paths:
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	both modified:   doc/index.rst
	both deleted:    gz_ros2_control_demos/config/diff_drive_controller_velocity.yaml
	deleted by us:   gz_ros2_control_demos/launch/diff_drive_example.launch.py
	added by them:   ign_ros2_control_demos/config/diff_drive_controller.yaml
	added by us:     ign_ros2_control_demos/config/diff_drive_controller_velocity.yaml
	added by them:   ign_ros2_control_demos/launch/diff_drive_example_namespaced.launch.py
	both modified:   ign_ros2_control_demos/urdf/test_diff_drive.xacro.urdf

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[christophfroehlich]

Tested on humble