Page Not Found page out of sync with the site

[mhauru]

If I go to e.g. https://turinglang.org/docs/tutorials/this-tutorial-does-not-exist I get a Page Not Found page, but it's not in dark mode, even though I've set dark mode for the rest of the site. I think some of the other styling might be a bit off as well, like the menu titles seem to shift a bit. Also, e.g. the link in the top bar to Tutorials points to https://turinglang.org/tutorials/00-introduction/, whereas it should point to https://turinglang.org/docs/tutorials/00-introduction/.

[penelopeysm]

Fixed by #575