Create guides for ROS2 launch and ROS2 parameters

[audrow]

In my understanding of where we are going with the ROS 2 documentation, guides are different than tutorials in that they are not necessarily examples for readers to follow along, but rather are meant to be concise references for doing specific things in ROS 2.

The following guides could be written for the ROS 2 documentation. Each checkbox is intended to be one guide, with sublists providing more detail about what content could be included in the guide.

ROS2 Launch

• Overview of Python launch files (Group the launch file tutorials #2180)
• Using parameters in launch files (Intermediate launch tutorial #2002)
  • Parameters from config file
  • Define parameters in a launch file
  • Pushing into a namespace
• Launching other launch files (Intermediate launch tutorial #2002)
  • Simple include
  • Remapping
  • Namespaces
• Comparing launch files written in YAML, XML, and Python (Add guide for using Python, XML, and YAML in launch files #1468)
  • Show all three examples
    • Node
    • Namespace
    • Set parameter
    • Args
    • Substitution
• Using substitution in launch files (Tutorial about using event handlers and substitutions in launch files #2094)
  • Text
  • Environment variable
  • Executable

ROS2 Parameters

• Parameter files (Intermediate launch tutorial #2002)
  • Wildcards
  • Namespaces

[audrow]

Additional thoughts:

• Cover Python, XML, and YAML for all ROS2 launch guides
• Show parameter override precedence in Node action

[clalancette]

* Cover Python, XML, and YAML for all ROS2 launch guides

Ivan made a good start with that in this PR: #1109 . Probably we can just work on and expand that one.

[audrow]

It would also be good to have DDS troubleshooting across hosts.

[ros-discourse]

This issue has been mentioned on ROS Discourse. There might be relevant details there:

https://discourse.ros.org/t/gsod-2021-ros2-tutorials-on-tf2-and-ros2-launch/20490/1

[audrow]

It would be great to add how to use launch with a parameter file. I think a good spot for this is here: https://github.com/ros2/ros2_documentation/blob/rolling/source/Tutorials/Parameters/Understanding-ROS2-Parameters.rst#7-load-parameter-file.

[audrow]

All the items have been added!