Docs: Replace `airflow.sdk.definitions` to `airflow.sdk` for public facing docs

Currently the code lives in the following, which is right
```python
from airflow.sdk.definitions.dag import DAG as DAG
```

but we want users to just see `from airflow.sdk import DAG`

So we might have to write a Sphinx extension to not show first one and just second one (intended for public usage)