docs: code samples for sample, get, Series.round

[shobsi]

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docs: code samples for sample, get, Series.round (#295)

docs: code samples for DataFrame set_index, items (#295)
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Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• 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
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)
  • DataFrame.sample, Series.sample: https://screenshot.googleplex.com/kPy5swVACMeBhSo
  • DataFrame.get, Series.get: https://screenshot.googleplex.com/7hirn5oz2b4L6B3
  • DataFrame.set_index: https://screenshot.googleplex.com/3CXARrp5hwV6gau
  • DataFrame.items: https://screenshot.googleplex.com/bk3HAiXZQq3TYD9
  • Series.round: https://screenshot.googleplex.com/C9c4m84NWNMnAwS

Fixes internal issues 318011542 and 318011745 🦕