Review Japanese translation

[kristoff-it]

Ported the old Japanese translation to the new content format.

Despite watching lots of anime, I can't read Japanese so help from a native speaker would be appreciated both in terms of double checking that I didn't make errors while copy pasting, and also with translating parts of the website that were not translated at all.

This is the commit where I did all the work: f5ce4ba

[kawaken]

@kristoff-it Hello, I'm a native Japanese speaker. I'll review the contents of the commit.
It might take some time to translate the new sections, is that okay with you?

[andrewrk]

ようこぞ@kawaken

ゆっくりでください。私たちに大きな助かっていますよね。やるの人は僕だったら、十年ぐらいと思います😅

よろしくおねがいします！