Change how invalid pages are handled

[nocduro]

Right now invalid pages like https://rustref.com/invalid will hit the static file handler route, which has to check the filesystem for that file before returning 404. The origin server is not running on an ssd, so this is may be a little slower than it needs to be.

Not sure how to change it though. Maybe just have a route for each of favicon.ico and robots.txt? Or maybe Caddy should handle those files?