docs: document widget-tutorials layout; retarget notebooks to it

clarify: Colab T4 runs on GPU via PyTorch (not CPU)

install with -U so the latest rc always installs (was keeping cached rc9)

default Colab to free T4 GPU

update Show4DSTEM Colab to unified load/Show4DSTEM API + TestPyPI install

nb: pip from electronmicroscopy/quantem@dev (real github branch with from_tensor)

nb: Colab metadata pre-selects T4 GPU runtime

docs(notebook): add show4dstem_colab.ipynb for one-click Colab workshop