DOC: demonstrate "string timestamp" functionality in user guide and case studies

[venaturum]

In v2.4.0 the ability to pass string representations of timestamps into Stairs.clip, Stairs.mask, Stairs.where was introduced. This is so users can do this

my_stepfunction.clip("2020", "2021")

instead of my_stepfunction.clip(pd.Timestamp("2020"), pd.Timestamp("2021"))

There are opportunities to utilise this new feature in examples found in the user guide and case studies. Perhaps the best approach is, for each page, use the longer version first, then shorter version in subsequent code, noting that the feature is being used.