📝 Add docs for extra models

docs/tutorial/extra-models.md

@@ -0,0 +1,45 @@
+Continuing with the previous example, it will be common to have more than one related model.
+
+This is especially the case for user models, because:
+
+* The **input model** needs to be able to have a password
+* The **output model** should do not have a password
+* The **database model** would probably need to have a hashed password
+
+!!! danger
+    Never store user's plaintext passwords. Always store a secure hash that you can then verify.
+
+## Multiple models
+
+Here's a general idea of how the models could look like with their password fields and the places where they are used:
+
+```Python hl_lines="8 10 15 21 23 32 34 39 40"
+{!./tutorial/src/extra-models/tutorial001.py!}
+```
+
+!!! warning
+    The supporting additional functions are just to demo a possible flow of the data, but they of course are not providing any real security.
+
+## Reduce duplication
+
+Reducing code duplication is one of the core ideas in **FastAPI**.
+
+As code duplication increments the chances of bugs, security issues, code desynchronization issues (when you update in one place but not in the others), etc.
+
+And these models are all sharing a lot of the data and duplicating attribute names and types.
+
+We could do better.
+
+We can declare a `Userbase` model that serves as a base for our other models. And then we can make subclasses of that model that inherit its attributes (type declarations, validation, etc).
+
+All the data conversion, validation, documentation, etc. will still work as normally.
+
+That way, we can declare just the differences between the models (with plaintext `password`, with `hashed_password` and without password):
+
+```Python hl_lines="8 14 15 18 19 22 23"
+{!./tutorial/src/extra-models/tutorial002.py!}
+```
+
+## Recap
+
+Use multiple Pydantic models and inherit freely for each case. You don't need to have a single data model per entity if that entity must be able to have different "states". As the case with the user "entity" with a state including `password`, `password_hash` and no password.