docs: code samples for Series.dot and DataFrame.dot

[shobsi]

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
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• Appropriate docs were updated (if necessary)
  • Series.dot: https://screenshot.googleplex.com/49WfXhzMKBZmhxn
  • DataFrame.dot: https://screenshot.googleplex.com/7dFBVS4k64QVuX7

Fixes internal issue 312553692 🦕

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[ashleyxuu]

Thanks for adding clear code samples. I love how detailed it is to have cases for DataFrame with a Series/DataFrames, and user-defined index.