Update ROS2 Documentation for Launch to encourage declarative practices

[emersonknapp]

Work tracking

• Reduce repetition, use literalinclude to include large blocks especially ones that are repeated in explanations
  • Done in Pull all example launchfiles into separate file with literalinclude ros2/ros2_documentation#5155
• Recommend and apply the .launch extension in all uses except references to actual existing demo files in other packages.
  • May not do this, in favor of IncludeLaunchDescription - request name without suffix/extension ros2/launch#849
• Move trivial launch examples in tutorials to XML and/or YAML, instead of Python
• Remove use of PythonLaunchDescriptionSource as a user-facing thing in all Python cases to match XML/YAML usage and move toward a less opinionated default launch usage pattern

[emersonknapp]

ros2/ros2_documentation#5156 installs all launchfiles