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/jazzy/pr-457
Your branch is up to date with 'origin/jazzy'.

You are currently cherry-picking commit a7d5a40.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	renamed:    gz_ros2_control_demos/config/diff_drive_controller_velocity.yaml -> gz_ros2_control_demos/config/diff_drive_controller.yaml
	modified:   gz_ros2_control_demos/launch/diff_drive_example.launch.py
	new file:   gz_ros2_control_demos/launch/diff_drive_example_namespaced.launch.py
	modified:   gz_ros2_control_demos/urdf/test_diff_drive.xacro.urdf

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   doc/index.rst

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]

waiting for #465