text: Title: [DOC] detection notebook - move plotting functions to `annotation.plotting`
  Body: In the detection notebook 7 (added in https://github.com/sktime/sktime/pull/7284), @alex-jg3 has created a number of plotting functions that would be nice to have generally.
  
  I think we should refactor the plotting functions and move them to `annotation.plotting`. There are also some existing functions in the latter, it might be useful to see if they can be merged.
  
  As a recipe:
  
  1. look at every block in the notebook 7, where `matplotlib` is used and something plotted
  2. try to turn this into a function `plot_something(param1, param2, ax=None)` (where if `ax` is passed, no new `ax` is created but that is used)
  3. put the function in `annotation.plotting`, and have the notebook import it from there instead of defining the plot in the notebook.