Add Demos for SDF

[Amronos]

Adds demos for SDF. Linked to #ros2_control/1763.
These demos should help in testing the PR.

A couple of things I want an opinion on:

1. Should using URDF or SDF be an argument of the launch file?
2. What should be the name of the launch file?
3. I have put the SDF files in a new sdf directory is that okay or should something else be done?

To test the diff_drive demo for SDF:
ros2 launch gz_ros2_control_demos diff_drive_example_sdf.launch.py
ros2 run gz_ros2_control_demos example_diff_drive
The simulation should show a behaviour similar to the diff_drive demo for URDF.

[Amronos]

@ahcorde Can I get an initial review?

[ahcorde]

Suggested change

	# Copyright 2022 Open Source Robotics Foundation, Inc.
	# Copyright 2024 Open Source Robotics Foundation, Inc.

[ahcorde]

you don't need to do that, the sdf file doesn't have any xacro tag. You should read the file as it's

[ahcorde]

you should be able to use something like from launch.substitutions import FileContent

[Amronos]

@ahcorde I decided it would be better for the description format to be an argument of the launch file. I did need to use an OpaqueFunction for this (please tell me if there is a simple way), have a look at ed7f047.
Another option would be not using SDF at all in the launch files and instead just having them be present in the sdf directory.

[bmagyar]

If the purpose of the demo is to have a urdf and sdf description separately demonstrated but in the same use case, I think the separate argument makes sense.

[ahcorde]

Do you mind to add some docs to the README.md ?

[ahcorde]

There are conflicts, do you mind to fix it ?

[Amronos]

If everything looks good now, I will also add SDF versions for the other demos.

[ahcorde]

While I was compiling the documentation of this PR I noticed that the setup is wrong. This PR should configure the docs file properly.

also If you run the documentation command there are some errors

[ahcorde]

There are some issues:

gz_ros2_control_demos/launch/ackermann_drive_example.launch.py:39:21: E128 continuation line under-indented for visual indent
gz_ros2_control_demos/launch/ackermann_drive_example.launch.py:40:21: E128 continuation line under-indented for visual indent
gz_ros2_control_demos/launch/ackermann_drive_example.launch.py:91:9: E222 multiple spaces after operator
gz_ros2_control_demos/launch/diff_drive_example.launch.py:39:21: E128 continuation line under-indented for visual indent
gz_ros2_control_demos/launch/diff_drive_example.launch.py:40:21: E128 continuation line under-indented for visual indent

[ahcorde]

is the robot moving ? I can't move the diff_drive or ackermann examples

[Amronos]

is the robot moving ? I can't move the diff_drive or ackermann examples

Yes, the robot does move for me upon running ros2 run gz_ros2_control_demos example_diff_drive for diff_drive or ros2 run gz_ros2_control_demos example_ackermann_drive for ackermann.
Make sure you have the sdformat_urdf package installed and are using a version of hardware_interface that has this commit.

[ahcorde]

The issue that I mentioned is unrelated to this PR, we can merge this.