Docs: Tutorial: Image Classification VGG

[chr5tphr]

• added support for tutorial ipynb using nbsphinx: my initial plan was
  to add myST markdown notebooks, however, these do not work with binder
  and colab, since nothing except notebooks in github repositories can
  be used with these services
• added buttons for colab and binder to the tutorial notebooks; I would
  have preferred to use pyodide, however, torch does not yet work with
  pyodide
• added an image classification tutorial using VGG11
• two new documentation requirements: nbsphinx and ipykernel
• the tutorial ipython notebooks are executed when building the
  documentation