[ENH] Remove yfinance from notebooks and examples

[fkiraly]

yfinance regularly downloads data from the internet to run the notebook examples.

Instead, the examples and notebooks should not rely on downloads, but, optimally, on a small csv example file that ships with the package for the purpose of examples. This should be "hidden" behind a load_stockdata loader function or similar.

There are already csv files in the cookbook folder which could be used for that, albeit they may have to be shortened.

Once this is done, it should be possible to entirely remove the dependency on yfinance.

There is already a PR that started to do this - #678 - but it got abandoned and also made some unrelated changes.