Rework the documentation

docs/src/db_tortoise_model.py

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+from fastapi_users import models
+from fastapi_users.db import TortoiseBaseUserModel
+from tortoise.contrib.pydantic import PydanticModel
+
+
+class User(models.BaseUser):
+    pass
+
+
+class UserCreate(models.BaseUserCreate):
+    pass
+
+
+class UserUpdate(models.BaseUserUpdate):
+    pass
+
+
+class UserModel(TortoiseBaseUserModel):
+    pass
+
+
+class UserDB(User, models.BaseUserDB, PydanticModel):
+    class Config:
+        orm_mode = True
+        orig_model = UserModel