These are the references :-

http://wiki.ros.org/Documentation

https://answers.gazebosim.org/questions/

https://askubuntu.com/

https://portal.e-yantra.org/storage/xyBeoIQDWX_vb/home/home.html

https://stackoverflow.com/

https://www.w3schools.com/

https://www.mkdocs.org/#getting-started

https://github.com/ros/ros_tutorials

http://docs.ros.org/en/melodic/api/moveit_tutorials/html/index.html