Replacing "Windows only: ..." with the .. availability:: Windows directive in ctypes doc.

[junkmd]

Documentation

This is a follow-up to gh-126615.
In the ctypes documentation, the supported platforms were previously described in plain text.
This will be replaced with a more modern approach using directives.

Linked PRs

• gh-127082: Replace "Windows only: ..." with the .. availability:: Windows directive in ctypes doc. #127099