docs: add code samples for Series.{between, cumprod}

[ashleyxuu]

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  • Series.between(): https://screenshot.googleplex.com/BhHpZsL7S9d3FsG
  • Series.cumprod(): https://screenshot.googleplex.com/7o7gDNwJZEWst84

Fixes #<issue_number_goes_here> 🦕