Add cart-pole demo (backport #289)

[mergify]

Similar to ros-controls/gazebo_ros2_control#172

The difference to gazebo_ros2_control is that it also works with position command interface of the cart. gz_ros2_control does not directly set the position but implements a position controller with velocity output:

gz_ros2_control/gz_ros2_control/src/gz_system.cpp

Lines 632 to 651 in 81dca71

	} else if (this->dataPtr->joints_[i].joint_control_method & POSITION) {
	// Get error in position
	double error;
	error = (this->dataPtr->joints_[i].joint_position -
	this->dataPtr->joints_[i].joint_position_cmd) * *this->dataPtr->update_rate;
	// Calculate target velcity
	double target_vel = -this->dataPtr->position_proportional_gain_ * error;
	auto vel =
	this->dataPtr->ecm->Component<sim::components::JointVelocityCmd>(
	this->dataPtr->joints_[i].sim_joint);
	if (vel == nullptr) {
	this->dataPtr->ecm->CreateComponent(
	this->dataPtr->joints_[i].sim_joint,
	sim::components::JointVelocityCmd({target_vel}));
	} else if (!vel->Data().empty()) {
	vel->Data()[0] = target_vel;
	}

---

This is an automatic backport of pull request #289 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 27af210 has failed:

On branch mergify/bp/humble/pr-289
Your branch is up to date with 'origin/humble'.

You are currently cherry-picking commit 27af210.
  (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:
	modified:   doc/index.rst

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	added by them:   ign_ros2_control_demos/launch/pendulum_example_effort.launch.py
	added by them:   ign_ros2_control_demos/launch/pendulum_example_position.launch.py
	added by them:   ign_ros2_control_demos/urdf/test_pendulum_effort.xacro.urdf
	added by them:   ign_ros2_control_demos/urdf/test_pendulum_position.xacro.urdf

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