Create ROS2 documentation for smach

[martincerven]

It would be nice to have up to date docs for ROS2 smach. Without it, new people will have zero incentive to use it with ROS2, since a lot of things changed from ROS1, and if they can't run examples with copy/paste from docs, they will move to some other, more maintained packages.

It's also not super obvious how to write docs for ROS2 packages, but after some digging the docs should live inside package repo and be automatically built with ROS buildfarm instead of editing external wikis, and be accessible here: https://docs.ros.org/en/rolling/p/smach/

I could start digging into/writing docs as soon I get more familiar with smach on ROS2, but it would be nice if other users/maintainer gave feedback/contributed.

Here are some useful links:

• new ROS2 package design:
  • https://design.ros2.org/articles/per_package_documentation.html
• https://docs.ros.org/en/humble/How-To-Guides/Documenting-a-ROS-2-Package.html#package-documentation-overview
• https://discourse.ros.org/t/ros-2-package-documentation/14569/3
• guide for creating ROS2 docs:
  • https://discourse.ros.org/t/new-guide-on-docs-ros-org-writing-per-package-documentation/36743