Support docs in multiple languages

I know we had the docs translated to Japanese in the past and, although the initial work seems to have been good, it did not work out because the Japanese docs quickly got out of date with the English docs, and we eventually dropped it.

https://github.com/pytest-dev/pytest/issues/6900#issuecomment-598727326 made me think if we should perhaps pursue this again, but this time using a framework/overflow where the translated docs would happen outside/independently of pytest's own repository. I'm sure there are tools to help it out as well (didn't take the time to really look for it though).

Start with the discussion linked from issue 6900, especially comment 598727326, and review this issue's prior comments for the translation approach that was considered. Research frameworks or workflows that keep translated documentation outside pytest's repository and prevent it from drifting out of date. Done would be an agreed, maintainable approach rather than a specific file change.