SDL3 documentation integrated — multilingual package available

[victor_18993]

The SDL3 backend documentation is now integrated in r12239. 😊
I have also attached the multilingual documentation package so it is easier to read and review without having to build anything locally.

https://files.simutrans-germany.com/upload/simutrans-sdl3-documentation-package.zip

Available languages:
🇬🇧 English
🇪🇸 Spanish
🇩🇪 German
🇯🇵 Japanese
🇰🇷 Korean
🇷🇺 Russian
🇫🇷 French
🇧🇷 Portuguese (Brazil)
🇨🇳 Chinese (Simplified)
🇹🇼 Chinese (Traditional)
🇨🇿 Czech

The package includes the documentation in PDF and wiki-source form, together with the validation summary and hashes.
There are also two small auxiliary fixes attached separately for review — registry and rendering/stylesheet — but these are only proposals and are not integrated.

The Korean edition is usable, although four particle choices are still marked for native-speaker review.
Thanks again to everyone who has been helping with SDL3 testing and documentation.