add some questions from the tutorial

user-guide/faq.md

@@ -2,4 +2,20 @@
 
 :::{note}
 Feel free to add any Ginkgo related question here
-:::
+:::
+
+## Questions from the Tutorial
+
+- s.7.1. reorder solver apply to show linear system, rename/reorder vectors
+  - how is S stored? as A^-1?
+  - use solver as preconditioner?
+  - combination: are the entries of C calculated?
+- s.12 duplicate entries?
+  - difference between device_matrix_data and matrix_assembly_data
+  - meaning of sum_duplicates function?
+- s.15.4 possible to combine criterion 1 and 2?
+  - what is residual norm criteria?
+- left or right preconditioning?
+- why output parameters (e.g. apply)?
+- why is the `.on` required for the factory?
+- cheatsheet???