Redo statvar examples

[kmoscoe]

Rewrite stat var section to use a correct measuredProperty and statType, and add other fields, like measurementDenominator. Update CSV examples.

[gemini-code-assist]

Code Review

This pull request updates the custom data documentation (custom_dc/custom_data.md) to revise the example statistical variables representing UN WHO cigarette smoking prevalence. The review feedback correctly identifies that Female and Male are not valid Class types in Data Commons and therefore cannot be used as populationType. The reviewer suggests reverting populationType to Person and using gender as a constraint property instead. Additionally, the feedback addresses a property name mismatch, a spelling typo ("prevelance" vs "prevalence"), and provides corresponding updates to the explanatory text.