[FEATURE]: Make samples explicit and brief speed guide

[MartinSmeyer]

Is your feature request related to a problem? Please describe.
Required samples are highly scene dependent. We have hidden default values that often take unnecessarily long.

Describe the solution you'd like
We should make the samples explicit in examples again so that users have guidelines and the option to iterate faster.

Describe alternatives you've considered
A quickmode option. Or low default amount of samples, but this might lead to smoothed high-frequency artifacts of the denoiser depending on the scene. We don't want to assume that this is okay for every learning task even though it is for some.