Add xeus-robot to the Debugger documentation

[jtpio]

Since the recently announced xeus-robot kernel also supports the JupyterLab visual debugger, we should consider adding it to the documentation:

https://jupyterlab.readthedocs.io/en/stable/user/debugger.html?#debugger

Blog post about xeus-robot: https://blog.jupyter.org/robotic-process-automation-with-jupyterlab-713b5630e457

[jtpio]

This would require following the guide for writing documentation: https://jupyterlab.readthedocs.io/en/stable/developer/contributing.html#writing-documentation

And edit this file: https://github.com/jupyterlab/jupyterlab/blob/master/docs/source/user/debugger.rst