Improve documentation #708

We have two diferrent places where the documentation is written:

• In the README.md of each repository
  • ros_gz_bridge
  • ros_gz_image
  • ros_gz_interfaces
  • ros_gz_sim
  • ros_gz_sim_demos
• In the official Gazebo documentation [1]

The idea is to move all the documentation to the Gazebosim docs website [2]

[1] https://gazebosim.org/docs/latest/ros2_overview/
[2] https://github.com/gazebosim/docs

FYI @j-rivero