Documentation improvement: document terminal commands for the usage of the backends

[SimeonEhrig]

Depending on the used backend, the way how to measure and view profile and trace data changes. For example, an application, which is annotated with score-p does automatically generate a profile file, if it is executed. In contrast of it, the nvidia compiler needs to be called manually to profile an application, e.g. nsys -o trace.prof ./app --param foo && nsys-gui trace.prof.

It would be really helpful to have a small get started guide for each backend, to know how to do a profile and view it.