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

[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.

[mhilbrunner]

But is there any reason not to have it in the 4.4 docs?

There could be an argument to keep guidelines for a version to the state that applied to that version, i.e., to have Godot 3 docs reflect Godot 3 guidelines. I think that argument is stronger for major versions however, I don't think the complexity is worth it for minor versions.

And kinda moot anyway considering we mostly decided to eventually move to one (current) set of contributor documentation and to move it out of the (user-facing) docs. I see no harm in cherrypicking these (doesn't seem too critical either though).

[skyace65]

Thanks!