[Docs]: Show example of how to aggregate percentage changes in DataTable

[archiewood]

Request Type

Explanation Improvement (make a description clearer for the reader of the docs)

Description

Need to use custom weight column.

https://evidencedev.slack.com/archives/C0239KTKHH7/p1727458101810239

[nickfanion]

I asked the original question on Slack. Percent change aggregations need totalAgg=weightedMean and weightCol set to the "old value" (for example, if comparing sales to sales_year_ago, weightCol should be sales_year_ago). The current docs always use the "new value" (sales) when a percent change subtotal example is provided.