Update composition tutorials for new tooling.

[mjcarroll]

This is to bring the tutorial inline with the enhancements made in the Dashing release.

WIP until a launch component demo is available.

Connects to #167

[hidmic]

LGTM but for a few nits!

[mjcarroll]

Resolved in commit before I realized that DCO isn't enforced here.

[christianrauch]

I think the documentation should contain a section to explain how to add the component to the ament index, i.e. how to prevent: Failed to load component: Could not find requested resource in ament index.

Edit: It turns out, you need to start the component process (ros2 run rclcpp_components component_container) from the workspace that contains the component. This should maybe be mentioned as it is currently a limitation. E.g. you cannot load components from different workspaces into the same process.

[mjcarroll]

@christianrauch Yes, I'll note this, but the container process needs to have visibility into the workspace that it is trying to load components from, otherwise they won't appear in the index. Thanks for the feedback.