Document [optional] in Installation section about other Qt backends

[psobolewskiPhD]

🧰 Task

In the section where other backends are discussed -- alternative to [all]
https://napari.org/stable/tutorials/fundamentals/installation.html#choosing-a-different-qt-backend
--we should mention [optional].
e.g.
pip install napari[pyqt6, optional]
will be identical to [all] except using pyqt6 instead of pyqt5.
(pending napari/napari#7511)