[PEP 695] Document Python 3.12 type parameter syntax (#17816)

Provide examples using both syntax variants, and give both of the syntax
variants similar prominence. It's likely that both syntax variants will
continue to be widely used for a long time.

Also adjust terminology (e.g. use 'type parameter' / 'type argument'
more consistently), since otherwise some descriptions would be unclear.

I didn't update examples outside the generics chapter. I'll do this in a
follow-up PR.

Work on #17810.

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Co-authored-by: Jelle Zijlstra <[email protected]>
Co-authored-by: Brian Schubert <[email protected]>