Add database structure docs

thanks to @desperateCoder for drawing an er diagram

Signed-off-by: Julius Härtl <jus@bitgrid.net>

docs/API.md

@@ -1,9 +1,10 @@
 
-The REST API provides access for authenticated users to their data inside the Deck app.
+The REST API provides access for authenticated users to their data inside the Deck app. To get a better understand of Decks data models and their relations, please have a look at the  [data structure](structure.md) documentation.
 
 # Prequisited
 
-All requests require a `OCS-APIRequest` HTTP header to be set to `true` and a `Content-Type` of `application/json`. The API is located at https://nextcloud.local/index.php/apps/deck/api/v1.0
+- All requests require a `OCS-APIRequest` HTTP header to be set to `true` and a `Content-Type` of `application/json`.
+- The API is located at https://nextcloud.local/index.php/apps/deck/api/v1.0
 
 ## Naming
 

docs/structure.md

@@ -0,0 +1,6 @@
+## Database structure
+
+Deck stores most of its data inside of the database. The structure and relationships between entities is documented in the following ER diagram:
+
+![Screenshot](resources/er-diagram.jpg)
+

mkdocs.yml

@@ -9,4 +9,6 @@ pages:
     - API documentation:
         - REST API: API.md
         - Nextcloud API: API-Nextcloud.md
+    - Developer documentation:
+        - Data structure: structure.md