Migrate user documentation away from Doxygen

[traversaro]

Doxygen documentation is not really accessible for most users, so it probably make sense to move documentation that want to use the plugins to another system, while keeping doxygen for people that want to check documentation of function signature or that kind of aspects.

In my opinion, a good trade-off between user friendly and low maintenance is just to write nicely written README.md for each plugin, as done for example for the gazebo_yarp_robotinterface in https://github.com/robotology/gazebo-yarp-plugins/tree/master/plugins/robotinterface, and link them from the main README. This also gives for free version selection by setting the tag in the branch/tag selector of GitHub.