Restructure the examples as an individual packages

[bailaC]

Hello All,

while working with examples, I found it would be better if we

1. Keep individual examples complete in itself.
2. Each example should contain the proper folder structure
3. While merging any Pull Request, it MUST have the Readme file, which MUST include below points but not limited to,

• How to run the example (Detailed step)
• Expected command output (if any)
• Expected simulation output (rviz, Gazebo etc. if any)

The basic idea behind all the above points is,

1. Any example can be tried independently.
2. All the required files/folders will be inside a "Single" folder. Users need not to concern about any other files/folders.
3. Maintenance will be much cleaner.

These are just my thoughts. Just want to make life easier for the Beginners. Please feel free to add your thoughts.

[bailaC]

I also propose to have Readme files for each example in their respective folder. So that the ros2_control_demos Readme file stays clean.

Any thoughts @destogl , @bmagyar , @olivier-stasse

[destogl]

Everything is placed into #231