add docs

doc/db/boat.mermaid

@@ -0,0 +1,69 @@
+classDiagram
+    class Boat {
+        +int id
+        +string name
+        +int amount_seats
+        +int location_id
+        +int owner
+        +int year_built
+        +string boatbuilder
+        +bool default_shipmaster_only_steering
+        +bool convert_handoperated_possible
+        +string default_destination
+        +bool skull
+        +bool external
+        +bool deleted
+    }
+
+    class Location {
+        +int id
+        +string name
+    }
+
+    class Boathouse {
+        +int id
+        +int boat_id
+        +string aisle
+        +string side
+        +int level
+    }
+
+    class BoatDamage {
+        +int id
+        +int boat_id
+        +string desc
+        +int user_id_created
+        +datetime created_at
+        +int user_id_fixed
+        +datetime fixed_at
+        +int user_id_verified
+        +datetime verified_at
+        +bool lock_boat
+    }
+
+    class BoatReservation {
+        +int id
+        +int boat_id
+        +date start_date
+        +date end_date
+        +string time_desc
+        +string usage
+        +int user_id_applicant
+        +int user_id_confirmation
+        +datetime created_at
+    }
+
+    class User {
+      ...
+    }
+
+    Boat "*" -- "1" User : owner
+    Boat "*" -- "1" Location
+    Boathouse "*" -- "1" Boat
+    BoatDamage "*" -- "1" Boat
+    BoatDamage "*" -- "1" User : created_by
+    BoatDamage "*" -- "0..1" User : fixed_by
+    BoatDamage "*" -- "0..1" User : verified_by
+    BoatReservation "*" -- "1" Boat
+    BoatReservation "*" -- "1" User : applicant
+    BoatReservation "*" -- "0..1" User : confirmed_by