Update source/handbook/finance/operating-metrics/index.html.md

@tayloramurphy @wzabaglio @pmachle please review and adjust where y'all see fit.

Also, off topic but I really like the idea of conversational development in an MR.

My goal here is to separate out how we view subscription retention vs account or parent retention. I think it is meaningful for cohorting to see subscription based off of new customers and account/parent based off of new or existing customers

The way I envision subscription cohorts and retention is based off of the first chart in this view: https://modeanalytics.com/modeanalytics/reports/b1e301e40968/runs/5100b626467c