Style: Integrate new #pragma once syntax

[Repiteo]

• See Replace header guards style with #pragma once godot#102298

With #pragma once being the new stylistic syntax utilized in the main repo, this aims to update the docs to reflect that

[tetrapod00]

Suggested change

	``#ifdef``-based include guards
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	``#ifndef``-based include guards
	~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

[tetrapod00]

Suggested change

	Starting with 4.5, all files now use the ``#pragma once`` directive, as they
	improve readability and declutter macros. Use of ``#ifdef``-based include
	guards are now actively discouraged.
	Starting with Godot 4.5, all files now use the ``#pragma once`` directive, as they
	improve readability and declutter macros. Use of ``#ifndef``-based include
	guards are now actively discouraged.

Both suggestions optional.

These are technically ifndef-based not ifdef-based, right? But maybe calling them colloquially "ifdef-based" still makes sense?

[skyace65]

I know technically this is about changes in 4.5. But is there any reason not to have it in the 4.4 docs? I don't do coding for the engine so maybe I'm off here, but I wouldn't expect people to automatically assume they need to use the master branch for engine development.