Add RustDesk Client documentation

[StephanStS]

See MichaIng/DietPi#7992: A documentation part to https://dietpi.com/docs/software/remote_desktop/ is needed.

[Joulinar]

@StephanStS do you like to create Docs manually or should wen ask Copilot to do a first initial PR?

[StephanStS]

@StephanStS do you like to create Docs manually or should wen ask Copilot to do a first initial PR?

I would like to try this and learn at this point.