apply black formatting to chapters 08 to 16

chore - apply black formatting on chapter 02 to 07

increase the timeout cell execution to 300 seconds

updates to fix arviz trace plots & remove unused imports

in chapter 2, update Code 2.8 to use custom written algo for Metropolis

in chapter3, use seaborn to plot Code 3.24 / hist plot with arviz not working properly

remove the first markdown cell in chapter 16

remove the first markdown cell in chapter 8

update README

remove the "Chapter" from the title of the notebooks

delete data folder (not needed anymore)

in github action make sure to install the rethinking package

move dataset loading and posterior sampling to a local package

remove docs folder (not needed anymore as rendering using juypter-book)

update the levels of markdown cells