Add C++ Action tutorials

[jacobperron]

Follow-up from #166. We currently have Python tutorials, but there are only place-holder pages for the C++ tutorials.

Similar to the existing Python tutorials, the code can live in the action_tutorials package. To avoid maintaining multiple copies of the tutorial code, we might consider moving the entire tutorial into the action_tutorials package and reference it from the ROS 2 documentation.

[jacobperron]

@maryaB-osr Let me know what you think about the existing Action tutorials and if you have plans for refactoring. I think it would be nice to do the C++ tutorials (which I can do) before Foxy.

[maryaB-osr]

@jacobperron I do have an idea for refactoring, let me put that together and then you can fill in the C++ code based on the new structure

[clalancette]

This was done in #902, so closing this out.