In index, change the link to download Englich version of html or epub to i18n version of html or epub

[shutaozhenzhen]

Your Godot version:

Issue description:

I want to download Chinese version godot epub doc, but I can not find where to download it.
URL to the documentation page (if already existing):
https://docs.godotengine.org/zh-cn/4.x/

[tetrapod00]

I don't think we currently build translated versions of the offline docs, see #6176 (comment)