📝 Add docs annotations for source examples

docs_src/tutorial/create_db_and_table/annotations/en/tutorial003.md

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+1. Import `Optional` from `typing` to declare fields that could be `None`.
+2. Import the things we will need from `sqlmodel`: `Field`, `SQLModel`, `create_engine`.
+3. Create the `Hero` model class, representing the `hero` table in the database.
+
+    And also mark this class as a **table model** with `table=True`.
+
+4. Create the `id` field:
+
+    It could be `None` until the database assigns a value to it, so we annotate it with `Optional`.
+
+    It is a **primary key**, so we use `Field()` and the argument `primary_key=True`.
+
+5. Create the `name` field.
+
+    It is required, so there's no default value, and it's not `Optional`.
+
+6. Create the `secret_name` field.
+
+    Also required.
+
+7. Create the `age` field.
+
+    It is not required, the default value is `None`.
+
+    In the database, the default value will be `NULL`, the SQL equivalent of `None`.
+
+    As this field could be `None` (and `NULL` in the database), we annotate it with `Optional`.
+
+8. Write the name of the database file.
+9. Use the name of the database file to create the database URL.
+10. Create the engine using the URL.
+
+    This doesn't create the database yet, no file or table is created at this point, only the **engine** object that will handle the connections with this specific database, and with specific support for SQLite (based on the URL).
+
+11. Put the code that creates side effects in a function.
+
+    In this case, only one line that creates the database file with the table.
+
+12. Create all the tables that were automatically registered in `SQLModel.metadata`.
+
+13. Add a main block, or "Top-level script environment".
+
+    And put some logic to be executed when this is called directly with Python, as in:
+
+    <div class="termy">
+
+    ```console
+    $ python app.py
+
+    // Execute all the stuff and show the output
+    ```
+
+    </div>
+
+    ...but that is not executed when importing something from this module, like:
+
+    ```Python
+    from app import Hero
+    ```
+
+14. In this main block, call the function that creates the database file and the table.
+
+    This way when we call it with:
+
+    <div class="termy">
+
+    ```console
+    $ python app.py
+
+    // Doing stuff ✨
+    ```
+
+    </div>
+
+    ...it will create the database file and the table.