add 404 page

[EverlastingBugstopper]

Currently if you attempt to navigate to a page that does not exist, you get the default netlify 404 page. this should likely be replaced with the same or similar page to the 404 at https://apollographql.com/does-not-exist. I... tried to figure this out for a bit but couldn't quite get it to work. Happy to make a PR if anybody has ideas.