Document names of frontend action arguments

[christophebedard]

Resolves #404

This adds boilerplate for launch_ros docs along with a page to document the argument names for the frontend actions. The setup is similar to this page in the launch docs:

• https://github.com/ros2/launch/blob/rolling/launch/doc/source/architecture.rst
• https://docs.ros.org/en/rolling/p/launch/architecture.html

Tested with rosdoc2.

[christophebedard]

This is a draft PR because this is a WIP just to get feedback on the execution/solution.