Add 'Advanced'-Sections to the python-notebooks

[LuckyJosh]

Like I said in the issue on the pull plots:
Sorry, didnt get to comment in time.
I think it would be a good idea in general to have some
'further reading' material e.g. a section in each notebook that we do not discuss in the workshop but is in the materals for interested/advanced paticipants as an easy next step.

Originally posted by @LuckyJosh in #328 (comment)

Another good candidate would be the error propagation and fitting with minuit.

[jpwgnr]

Also maybe a section about unbinned vs binned fits similar to this