docs: code samples for Series.{argmax, argmin, is_monotonic_increasing, is_monotonic_decreasing}

[ashleyxuu]

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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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  • Series.argmin: https://screenshot.googleplex.com/82ogtuLDHwpqHar
  • Series.argmax: https://screenshot.googleplex.com/B8un8Y4hvJucyAS
  • Series.is_monotonic_decreasing/is_monotonic_increasing : https://screenshot.googleplex.com/AqdSYPLBDwjq5bq

Fixes internal issue #320714220 🦕