Clarify guidelines for spacing, detector positioning, and FOV extent in simulation setup.

[VasoKrg]

The current SIMPA documentation and example code (e.g., for segmentation-based simulations) do not provide clear guidance on how to choose and define key geometric parameters:

->spacing_mm

->device_position_mm

->element_spacing_mm

->field_of_view_extent_mm

it's unclear:
How to place the detector relative to the volume (e.g., on top, centered, offset?).
How to ensure the field of view is correctly aligned with the tissue geometry.
Whether spacing and element spacing need to satisfy specific conditions.
How these parameters influence accuracy, resolution, and performance.

[jgroehl]

Hi @VasoKrg. Thanks for raising this issue - the Segmentation-based volume creator is indeed not as developed as the model-based one and could use an overhaul.

Would you be happy to contribute the requested documentation and create a PR with the proposed changes?