DOC: add an example to `PeriodIndex.to_timestamp` to clarify the behavior in case len(index)<3
xref #56213

added an example to `PeriodIndex.to_timestamp` to clarify the behavior in case `len(index) < 3`
@MarcoGorelli, I added an example to clarify the behavior of `PeriodIndex.to_timestamp` in case `len(index) < 3`. Could you please take a look at this PR?