bpo-44957: Promote PEP 604 syntax in typing docs (GH-27833)

* Use "X | Y" instead of "Union" where it makes sense.
* Mention that "X | Y" is equivalent to "Union[X, Y]" in Union section.
* Remove "Optional[X]" as shorthand for "Union[X, None]" as the new
  shorthand is now "X | None".
* Mention that "Optional[X]" can be written as "X | None" in section
  about "Optional".

Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>