Replace yfinance with a small csv example file that ships with the package for the purpose of examples, accessed via a load_stockdata loader function.
`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.