Docs update

docs/assets/screenshots/CLI-data_explorer.gif

docs/assets/screenshots/qim3d-filters_example.png

docs/assets/screenshots/viz-orthogonal.gif

docs/assets/screenshots/viz-slicer.gif

docs/cli.md

docs/gui.md

docs/index.md

docs/io.md

docs/models.md

docs/processing.md

docs/releases.md

docs/stylesheets/extra.css

docs/utils.md

docs/viz.md

mkdocs.yml

qim3d/gui/annotation_tool.py

qim3d/io/downloader.py

qim3d/io/load.py

qim3d/io/save.py

qim3d/utils/img.py