Add an example with a passive joint (backport #172)

[mergify[bot]]

Adds a demo example with a passive joint: A pendulum hinged on the existing cart example.

I also added a hint that the physics of the passive joint are not rendered correct if position or velocity interface is used.

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This is an automatic backport of pull request #172 done by Mergify.

[mergify[bot]]

Cherry-pick of 7d5ec5dbad710d628bc14a82195c196f088621b8 has failed:

On branch mergify/bp/humble/pr-172
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Changes to be committed:
    new file:   gazebo_ros2_control_demos/launch/pendulum_example_effort.launch.py
    new file:   gazebo_ros2_control_demos/launch/pendulum_example_position.launch.py
    new file:   gazebo_ros2_control_demos/urdf/test_pendulum_effort.xacro.urdf
    new file:   gazebo_ros2_control_demos/urdf/test_pendulum_position.xacro.urdf

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

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