Add custom 404 page

[yochem]

Problem:
A non-existing link on the Neovim website shows the Github 404: https://neovim.io/nonexistingpage.

Solution:
Add basic 404 page. Github supports it if the site has a /404.html page: https://docs.github.com/en/pages/getting-started-with-github-pages/creating-a-custom-404-page-for-your-github-pages-site