Docs: Better image classification tutorial

- change the VGG tutorial to an image classification tutorial using
  VGG11 (with and without batch-norm) and ResNet18
- demonstrate Saliency Map and SmoothGrad first
- demonstrate EpsilonGammaBox and EpsilonPlusFlat
- demonstrate how to modify the stabilizer and epsilon of the built-in
  composites
- demonstrate different visualizations
- demonstrate how to modify the built-in composites using the
  rule-parameters and cooperative layer maps
- demonstrate a custom NameMapComposite
- demonstrate a custom LayerMapComposite
- add a ipynb-based table of contents for easier navigation and an
  overview
- change deprecated torchvision model argument `pretrained=False` to
  `weights=None` and `pretrained=True` to `weights='IMAGENET1K_V1'`