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
• [3.13] gh-127082: Replace "Windows only" with the availability: Windows in ctypes doc (GH-127099) #127144
• [3.12] gh-127082: Replace "Windows only" with the availability: Windows in ctypes doc (GH-127099) #127145

[Eclips4]

Thank you @junkmd!