add docs

doc/db/misc.mermaid

@@ -0,0 +1,30 @@
+classDiagram
+    class Log {
+        +int id
+        +string msg
+        +datetime created_at
+    }
+
+    class Notification {
+        +int id
+        +int user_id
+        +string message
+        +datetime read_at
+        +datetime created_at
+        +string category
+        +string action_after_reading
+        +string link
+    }
+
+    class Distance {
+        +int id
+        +string destination
+        +int distance_in_km
+    }
+
+    class User {
+      ...
+    }
+
+    %% Relationships
+    Notification "*" -- "1" User