docs: code samples for Series.where and Series.mask

[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
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated
  • Series.where: https://screenshot.googleplex.com/9XWHpMnwrzVPF9G
  • Series.mask: https://screenshot.googleplex.com/4cPvvzoVaVzoCDD

Fixes internal issue 310981880 🦕

[ashleyxuu]

Thanks for adding these code samples!