typing docs: Use soft deprecation

A while ago we introduced the concept of "soft deprecation" in PEP 387 (https://peps.python.org/pep-0387/#soft-deprecation): for things that we no longer recommend using, but which we aren't planning to remove in the near future.

The typing docs already list four classes of objects that are deprecated without a scheduled removal (https://docs.python.org/3.14/library/typing.html#deprecation-timeline-of-major-features):

• Pre-PEP-585 generic aliases (e.g. List)
• Text
• Hashable and Sized
• TypeAlias

I'd like to also soft-deprecate the following:

• Optional (obviated by PEP-604)
• NoReturn (prefer Never)
• typing.ForwardRef (deprecated alias for annotationlib.ForwardRef)
• types.UnionType (deprecated alias for typing.Union)
• Creating a Union using subscripting (Union[A, B] instead of A | B) (PEP-604)
• Creating a TypeVar, ParamSpec, or TypeVarTuple using the constructor directly (PEP-695)
• Directly inheriting from Generic (PEP-695)

None of those will be removable for many years if ever, but I think it's useful to have a clear statement in the docs that the newer syntax is preferred.

Linked PRs

• gh-154480

Start with the typing documentation section titled “Deprecation timeline of major features” at the linked Python documentation page, and compare its existing entries with the requested soft deprecations. Done means the documentation clearly identifies the listed older forms and recommends their newer alternatives; review linked PR gh-154480 first because work is already associated with it.