Update/docs/data architecture (#424)

* Update: add database schema section in data-architecture.md

* Update: change the database models image in data-architecture.md

* refacto: move images in the doc into docs/ directory

Author: LouisVdwl

database_models.jpg

@@ -1,6 +1,6 @@
 ## Database models
 
-![Database models](../database_models.jpg 'Database models')
+![Database models](database_models.jpg 'Database models')
 
 **User**
 Stores basic profile data for a user and relationships.
@@ -27,4 +27,7 @@ Stores data related to courses in Storyblok, copying the story id's. The `Sessio
 Stores relationship between a `User` and `Course` records, once a user has started a course. A users progress (`completed`) for the course is updated (`true`) when all related `SessionUser` records are `completed` for the related `Course`.
 
 **SessionUser**
-Stores relationship between a `User` and `Session` records, once a user has started a session. A users session progress (`completed`) for the session is updated (true) when the `/complete` endpoint is called.
+Stores relationship between a `User` and `Session` records, once a user has started a session. A users session progress (`completed`) for the session is updated (true) when the `/complete` endpoint is called.
+
+## Database Schema
+![Database schema](database_schema.png)