text: Title: [DOC] Add sktime.utils.plot_windows to the API Reference
  Body: #### Describe the issue linked to the documentation
  
  The utility function `sktime.utils.plot_windows` is not shown in the API reference. There is a similar "plot_windows" method defined here: https://www.sktime.net/en/v0.19.2/examples/forecasting/window_splitters.html However it does not have the same signature as the one defined at `sktime.utils.plot_windows`.
  
  #### Suggest a potential alternative/fix
  
  Add `plot_windows` to the API reference.
  