add docs

doc/db/logbook.mermaid

@@ -0,0 +1,38 @@
+classDiagram
+    class Logbook {
+        +int id
+        +int boat_id
+        +int shipmaster
+        +int steering_person
+        +bool shipmaster_only_steering
+        +datetime departure
+        +datetime arrival
+        +string destination
+        +int distance_in_km
+        +string comments
+        +int logtype
+    }
+
+    class LogbookType {
+        +int id
+        +string name
+    }
+
+    class Rower {
+        +int logbook_id
+        +int rower_id
+    }
+
+    class User {
+      ...
+    }
+
+    class Boat {
+      ...
+    }
+
+    Logbook "*" -- "1" Boat
+    Logbook "*" -- "1" User : shipmaster
+    Logbook "*" -- "1" LogbookType
+    Rower "*" -- "1" Logbook
+    Rower "*" -- "1" User