Added note about sample images, cleaned up directory structure

core/tutorial/doc/samples.md

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+# Sample images for sample code
+NICE-core supports PPM and PGM images out of the box without any additional
+dependencies.
+
+See the __samples__ folder for sample images that work with the sample code
+from the tutorials.
+
+## Grayscale sample images
+The grayscale samples are 8-bit PGM files.
+You can use them with the following sample programs:
+
+* Tutorial 02
+* Tutorial 03
+* Tutorial 04
+
+## Color sample images
+The color samples are 8-bit-per-channel PPM images.
+They work with the sample code from these tutorials:
+
+* Tutorial 01