Bug: 404 button not leading to home on nested pages

[caendesilva]

Currently the 404 page has a bug where it does not work on nested pages, due to the fact that the go home button links to the index.html page. This bug can be seen on the live site https://hydephp.github.io/docs/foo/bar.html, and since we can't calculate relative URLs like this we have a few options. We could make the button go back to the previous page or some kind of trickery.

But what I think I will do, unless anyone has any suggestions for how to do it better, is to make the link route to the app url if it is set, otherwise directly to the root path (href="/")