adapt the user_guide/concepts index

Author: luisaFelixSalles

doc/source/user_guide/concepts/index.rst

@@ -5,6 +5,16 @@ Concepts
 
 This section provides in-depth descriptions and explanations of DPF concepts, including terminology.
 
+
+.. toctree::
+   :maxdepth: 2
+   :hidden:
+   :caption: Concepts
+
+   concepts/concepts.rst
+   concepts/waysofusing.rst
+   concepts/stepbystep.rst
+
 .. grid:: 1 1 3 3
     :gutter: 2
     :padding: 2