Change include guards to #pragma once

[Cupcake6]

The Code Style Guide section on the website suggests to use traditional header guards, while the codebase has switched to using #pragma once

[eXpl0it3r]

The whole style guide needs an overhaul. It should generally show how to use clang-format & clang-tidy to automatically detect style issues.

[ChrisThrasher]

I’ve been working on SFML for about 5 years and have never reviewed the style guide so it could be very out of sync with the library. As Exploiter mentioned, a lot of our style rules are automatically enforced with clang-format and clang-tidy so we don’t really need to go into detail about many aspects of style. I don’t often hear about anyone reading that guide nor is it ever mentioned in PR reviews so perhaps it can be removed. I’m not sure it’s adding hardly any value these days.

If we do want to maintain a style guide, I’d rather it exist as a markdown file in SFML/SFML without a corresponding web page. Those who use the website are predominately people who want to use SFML as opposed to modify it. Those who do modify the code are interacting with us in Discord or in the main GitHub repo. Perhaps this information needs to live in the CONTRIBUTING.md file of SFML/SFML which hasn’t been touched in 7 years. We could be making better use of that file.

An overhaul is definitely warranted.