Docs: clarify ROS support; decide on the stub ROS backend #727

Raised by discussion #419: a user read the intro's design goal "being able to work with ROS but not be dependent on ROS" (docs/source/intro.rst) and concluded that RTB has a ROS interface, then could not find one.

Docs:

• Reword that bullet so it is clear RTB has no ROS dependency and is used by importing it inside a ROS or ROS 2 node, and that it provides no ROS API (see also ROS2 Support #332).
• Add a short "using RTB with a real robot / ROS" note: RTB for kinematics and planning, your own node for joint states and commands.

Code: src/roboticstoolbox/backends/ROS/ROS.py is a skeletal ROS 1 (rospy) stub: launch() instantiates a VelPub for a Panda and the publisher is mostly commented out. Decide whether to document it as experimental or remove it. The Dynamixel backend is similar ("Still under development" in its README).