docs(examples): add remaining selectors examples

[IndexSeek]

Description of changes

This adds examples for the remaining selectors except s.none(). I need to brainstorm for a good example of that one, unless anyone has ideas off the bat.

Selectors may still be my favorite Ibis feature, so I am happy to provide direct examples for using them. 😎

[cpcloud]

Thanks! The s.none() selector has a test case that could be useful, but it will probably not be possible to showcase its utility outside the context of a complex tabular operation like pivot_wider/pivot_longer