Added some documentation for multiarm support

[firesurfer]

https://github.com/UniversalRobots/Universal_Robots_ROS2_Driver/issues/680

We might want to consider adding also an example xacro file. I didnt do so as creating a testing such a configuration is quite a lot of work I wanted to avoid at the moment.

[fmauch]

Thanks for sharing this! I've actually planned to write a full-blown example documentation in https://github.com/UniversalRobots/Universal_Robots_ROS_Tutorials similar to https://github.com/UniversalRobots/Universal_Robots_ROS_Tutorials/tree/main/ur_example_dual_robot.

[firesurfer]

Oh that sounds great. I would nevertheless add a bit of basic information either into the description or the driver repository.