[Documentation] Improve QuantumModel docstrings

[chMoussa]

Closes #483

[awennersteen]

Super nice,
Maybe whilst we're here we can add usage examples to the QuantumModel class

    Example:
    ```python exec="on" source="material-block" result="json"
    from qadence import QuantumModel, QuantumCircuit, <some gates>
    

    ```
    ```

[jpmoutinho]

Very nice @chMoussa 👍