Translations for doc.qt.io

Documentation for Qt and tools have been only in English so far. But we also have a large user base who don't read English fluently, and who would benefit from a localized version of the documentation.

Translating even parts of our documentation to different languages manually is a significant task though, and would need to be repeated often to not become outdated. But nowadays, automatic translation tools like Google Translate and DeepL have vastly improved in recent years, so automatic translation of all of doc.qt.io in a good enough quality might be feasible.