docs: code samples for Series.{sum, mean, min, max}, astype

[shobsi]

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docs: code samples for astype common to DataFrame and Series (#280)

docs: code samples for Series.groupby and Series.{sum,mean,min,max} (#280)
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• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
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• Appropriate docs were updated (if necessary)
  • astype: https://screenshot.googleplex.com/5472hNUvrVgt2gS
  • Series.groupby:
    • https://screenshot.googleplex.com/5nyo5NY3WhDbd4S
    • https://screenshot.googleplex.com/boHMx3CqhDdHaAr
  • Series.sum: https://screenshot.googleplex.com/75Kr7sUKDx3sudR
  • Series.mean: https://screenshot.googleplex.com/7rzjPzCVqcWtwmv
  • Series.min: https://screenshot.googleplex.com/9pCCPWEn4CUZrr8
  • Series.max: https://screenshot.googleplex.com/6wUBrUGijrb96iP

Fixes #<issue_number_goes_here> 🦕