docs: code samples for Series.{add, replace, unique, T, transpose}

[shobsi]

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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.add: https://screenshot.googleplex.com/763p36yJKKvM5DY
  • Series.replace: https://screenshot.googleplex.com/9MHuQZnKakqjjJn
  • Series.unique: https://screenshot.googleplex.com/7BfuQE3bFcYASqu
  • Series.T: https://screenshot.googleplex.com/8cSYpwKXrYetsEg
  • Series.transpose: https://screenshot.googleplex.com/7mM2zBwxRiqfDUV

Fixes internal issue 317297573 🦕