Documentation: reorganized list of tutorials

Documentation/Tutorials/index.md

 \page Tutorials Tutorials
 
+## Basic topics
+
 1. [General concepts](./GeneralConcepts/tutorial_GeneralConcepts.md)
 2. [Arrays](./Arrays/tutorial_Arrays.md)
 3. [Vectors](./Vectors/tutorial_Vectors.md)
@@ -7,8 +9,14 @@
 5. [For loops](./ForLoops/tutorial_ForLoops.md)
 6. [Sorting](./Sorting/tutorial_Sorting.md)
 7. [Cross-device pointers](./Pointers/tutorial_Pointers.md)
-8. [Matrices](./Matrices/tutorial_Matrices.md)
-9. [Linear solvers](./Solvers/Linear/tutorial_Linear_solvers.md)
-10. [Segments aka sparse formats](./Segments/tutorial_Segments.md)
-11. [ODE solvers](./Solvers/ODE/tutorial_ODE_solvers.md)
-12. [Unstructured meshes](./Meshes/tutorial_Meshes.md)
+
+## Data structures
+
+1. [Matrices](./Matrices/tutorial_Matrices.md)
+2. [Segments aka sparse formats](./Segments/tutorial_Segments.md)
+3. [Unstructured meshes](./Meshes/tutorial_Meshes.md)
+
+## Numerical solvers
+
+1. [Linear solvers](./Solvers/Linear/tutorial_Linear_solvers.md)
+2. [ODE solvers](./Solvers/ODE/tutorial_ODE_solvers.md)