Add in new use cases for the flags chapter

[NathanKelly-ONS]

The flags chapter (located in the spark-analysis section and in raw-notebooks/flags) currently only has one use case for flags (age differences). I'd like to see some additional use cases for flags - for example if we assume somebody would like to conduct some sort of analysis on whether incidents are more common in summer than winter, an idea for a flag would be an "occurs_in_summer" and "occurs_in_winter" flag.

Or we could look at some of the cost variables, for example a flag for whether an incident costs more than £X amount.

Relatively simple flags, but I think they'd cover some of the use cases people would be looking at flags for.

Just to give a realistic example of a use case for flags, back when I was working on the COVID infection survey we were doing a logistic regression to see whether people were more likely to be hospitalised for COVID/Respiratory illnesses/Cardiovascular illnesses in winter vs summer, so we had to create a bunch of flags for covid_in_summer_2021, covid_in_summer_2022, covid_in_winter_2021, covid_in_summer_2022 (repeated for respiratory/cardiovascular). At the time my PySpark knowledge was horrendous so I did something incredibly inefficient (I think I did a for loop doing a bunch of groupbys and then joins), which is why I think we could really help the people that were in the same situation I was back then.