Create introduction documentation for newcomers

[micnekr]

We decided to add

1. Installation instructions in the main README.md
2. Instructions on how to use the packages and what the package does in the package README.md
3. Overviews to Git, ROS2 in RobotArm README.md, as well as links to examples
4. Eventually, the main README.md will have instructions on how to launch all required nodes

[micnekr]

Point 4) needs to be addressed once we create a launch script for all the required nodes

[JiaSenLaw]

I don't think we need a launch script just yet, especially since the nodes are not working together yet