add quetzal 4 steps

docs/quetzal_library/transport_modeling_intro.md

@@ -1,10 +1,27 @@
 # WORK IN PROGRESS | WIP
 This section is progress, we hope to provide a more comprehensive introduction to static macroscopic transport modeling soon.
 A complete introduction to the topic can be found in the book that inspired most of Quetzal's implementation.
+
 *de Dios Ortúzar, J., & Willumsen, L. G. (2024). Modelling transport. John wiley & sons*
 # Inputs
-WIP
+Work in progress
 # Outputs
-WIP
+Work in progress
 # 4 Step Structure
+Quetzal was inspired by the classical 4 step transport modeling and provides algorithms
 ![classical four step structure](/transport_modeling/steps_en.png)
+- The sequence of the four steps is based on :
+    - Data workflow constraints e.g., the outputs of the generation are used for the distribution
+    - Our understanding of the decision process 
+    - A calibration order that minimizes the need for feedbacks and saves time
+- It is flexible though:
+    - The travel times generated in the assignment are used in the upstream steps of distribution and modal split
+    - Two or more steps can be integrated in order to model our choices more accurately.
+
+- Although Quetzal provides the tools to build a standard four-stage model, we often use another framework to better model multi-modality and ensure consistency between the modal split and the assignment
+- *Generation* and *distribution* remain the same
+- The *pathfinder* step enumerates all the paths of the different modes, public and private
+- The *logit* step does the modal split and the weighting of the different itineraries
+- Retroaction *loops* are used between the steps
+![classical four step structure](/transport_modeling/steps_quetzal_loop_PR_en.png)
+