Add numbering and update README

README.md

images/jupyterlab_preview_pfeil.png

images/voila-basics.gif

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voila-examples/0_voila-basic.ipynb

voila-examples/1_voila-ipyvolume.ipynb

voila-examples/2_gridstack-example.ipynb

voila-examples/3_gridstack-scotch.ipynb

voila-examples/4_vuetify-bqplot.ipynb

voila-examples/5_vuetify-custom.ipynb