major changes to the figure-generating code

[murphyk]

• All figure URLs now have the form code.probml.ai/book1/x.y where x.y is figure number. All non-figure URLs have format code.probml.ai/book1/foo where foo is notebook name. (Oldfigures.probml.ai URLs point to code.probml.ai.)
• figures.probml.ai URLs used to be redirected to a cell in chapter-wise colab notebooks stored at https://github.com/probml/pml-book/tree/main/pml1/figure_notebooks. These notebooks were auto-generated from scripts stored at https://github.com/probml/pyprobml/tree/master/scripts or had pointers to other notebooks stored at https://github.com/probml/probml-notebooks. Now, each figure URL redirects to its own unique notebook, stored at https://github.com/probml/pyprobml/tree/master/notebooks/book1, with one notebook per figure.
• Figures that are generated by multiple notebooks point to a special dummy notebook with naming convention fig_xx_yy.ipynb , which contains links to the individual sub-figure notebooks.
• scripts folder and probml-notebooks repo are now deprecated. All the scripts that generated figures or referred in text are converted to notebooks and moved to notebooks/book1 folder. Several scripts that are not directly used to create figures but worked as helper scripts are moved to probml/probml-utils repo
• All torch code has been translated to JAX.