Docs update

docs/viz.md

mkdocs.yml

qim3d/__init__.py

qim3d/gui/data_explorer.py

qim3d/gui/iso3d.py

qim3d/gui/local_thickness.py

qim3d/img_examples/k3d.html

qim3d/img_examples/my_plot.html

qim3d/io/downloader.py

qim3d/io/save.py

qim3d/models/unet.py

qim3d/viz/img.py

qim3d/viz/k3d.py

requirements.txt

setup.py